Adding upstream version 4.19.249.upstream/4.19.249upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

12 files changed, 8410 insertions, 0 deletions

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diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
+        "http://www.w3.org/TR/html4/loose.dtd">
+        <html>
+        <head><title>A Tour Through TREE_RCU's Data Structures [LWN.net]</title>
+        <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
+
+           <p>December 18, 2016</p>
+           <p>This article was contributed by Paul E.&nbsp;McKenney</p>
+
+<h3>Introduction</h3>
+
+This document describes RCU's major data structures and their relationship
+to each other.
+
+<ol>
+<li>	<a href="#Data-Structure Relationships">
+	Data-Structure Relationships</a>
+<li>	<a href="#The rcu_state Structure">
+	The <tt>rcu_state</tt> Structure</a>
+<li>	<a href="#The rcu_node Structure">
+	The <tt>rcu_node</tt> Structure</a>
+<li>	<a href="#The rcu_segcblist Structure">
+	The <tt>rcu_segcblist</tt> Structure</a>
+<li>	<a href="#The rcu_data Structure">
+	The <tt>rcu_data</tt> Structure</a>
+<li>	<a href="#The rcu_dynticks Structure">
+	The <tt>rcu_dynticks</tt> Structure</a>
+<li>	<a href="#The rcu_head Structure">
+	The <tt>rcu_head</tt> Structure</a>
+<li>	<a href="#RCU-Specific Fields in the task_struct Structure">
+	RCU-Specific Fields in the <tt>task_struct</tt> Structure</a>
+<li>	<a href="#Accessor Functions">
+	Accessor Functions</a>
+</ol>
+
+<h3><a name="Data-Structure Relationships">Data-Structure Relationships</a></h3>
+
+<p>RCU is for all intents and purposes a large state machine, and its
+data structures maintain the state in such a way as to allow RCU readers
+to execute extremely quickly, while also processing the RCU grace periods
+requested by updaters in an efficient and extremely scalable fashion.
+The efficiency and scalability of RCU updaters is provided primarily
+by a combining tree, as shown below:
+
+</p><p><img src="BigTreeClassicRCU.svg" alt="BigTreeClassicRCU.svg" width="30%">
+
+</p><p>This diagram shows an enclosing <tt>rcu_state</tt> structure
+containing a tree of <tt>rcu_node</tt> structures.
+Each leaf node of the <tt>rcu_node</tt> tree has up to 16
+<tt>rcu_data</tt> structures associated with it, so that there
+are <tt>NR_CPUS</tt> number of <tt>rcu_data</tt> structures,
+one for each possible CPU.
+This structure is adjusted at boot time, if needed, to handle the
+common case where <tt>nr_cpu_ids</tt> is much less than
+<tt>NR_CPUs</tt>.
+For example, a number of Linux distributions set <tt>NR_CPUs=4096</tt>,
+which results in a three-level <tt>rcu_node</tt> tree.
+If the actual hardware has only 16 CPUs, RCU will adjust itself
+at boot time, resulting in an <tt>rcu_node</tt> tree with only a single node.
+
+</p><p>The purpose of this combining tree is to allow per-CPU events
+such as quiescent states, dyntick-idle transitions,
+and CPU hotplug operations to be processed efficiently
+and scalably.
+Quiescent states are recorded by the per-CPU <tt>rcu_data</tt> structures,
+and other events are recorded by the leaf-level <tt>rcu_node</tt>
+structures.
+All of these events are combined at each level of the tree until finally
+grace periods are completed at the tree's root <tt>rcu_node</tt>
+structure.
+A grace period can be completed at the root once every CPU
+(or, in the case of <tt>CONFIG_PREEMPT_RCU</tt>, task)
+has passed through a quiescent state.
+Once a grace period has completed, record of that fact is propagated
+back down the tree.
+
+</p><p>As can be seen from the diagram, on a 64-bit system
+a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout
+of 64 at the root and a fanout of 16 at the leaves.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why isn't the fanout at the leaves also 64?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Because there are more types of events that affect the leaf-level
+	<tt>rcu_node</tt> structures than further up the tree.
+	Therefore, if the leaf <tt>rcu_node</tt> structures have fanout of
+	64, the contention on these structures' <tt>-&gt;structures</tt>
+	becomes excessive.
+	Experimentation on a wide variety of systems has shown that a fanout
+	of 16 works well for the leaves of the <tt>rcu_node</tt> tree.
+	</font>
+
+	<p><font color="ffffff">Of course, further experience with
+	systems having hundreds or thousands of CPUs may demonstrate
+	that the fanout for the non-leaf <tt>rcu_node</tt> structures
+	must also be reduced.
+	Such reduction can be easily carried out when and if it proves
+	necessary.
+	In the meantime, if you are using such a system and running into
+	contention problems on the non-leaf <tt>rcu_node</tt> structures,
+	you may use the <tt>CONFIG_RCU_FANOUT</tt> kernel configuration
+	parameter to reduce the non-leaf fanout as needed.
+	</font>
+
+	<p><font color="ffffff">Kernels built for systems with
+	strong NUMA characteristics might also need to adjust
+	<tt>CONFIG_RCU_FANOUT</tt> so that the domains of the
+	<tt>rcu_node</tt> structures align with hardware boundaries.
+	However, there has thus far been no need for this.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>If your system has more than 1,024 CPUs (or more than 512 CPUs on
+a 32-bit system), then RCU will automatically add more levels to the
+tree.
+For example, if you are crazy enough to build a 64-bit system with 65,536
+CPUs, RCU would configure the <tt>rcu_node</tt> tree as follows:
+
+</p><p><img src="HugeTreeClassicRCU.svg" alt="HugeTreeClassicRCU.svg" width="50%">
+
+</p><p>RCU currently permits up to a four-level tree, which on a 64-bit system
+accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for
+32-bit systems.
+On the other hand, you can set <tt>CONFIG_RCU_FANOUT</tt> to be
+as small as 2 if you wish, which would permit only 16 CPUs, which
+is useful for testing.
+
+</p><p>This multi-level combining tree allows us to get most of the
+performance and scalability
+benefits of partitioning, even though RCU grace-period detection is
+inherently a global operation.
+The trick here is that only the last CPU to report a quiescent state
+into a given <tt>rcu_node</tt> structure need advance to the <tt>rcu_node</tt>
+structure at the next level up the tree.
+This means that at the leaf-level <tt>rcu_node</tt> structure, only
+one access out of sixteen will progress up the tree.
+For the internal <tt>rcu_node</tt> structures, the situation is even
+more extreme:  Only one access out of sixty-four will progress up
+the tree.
+Because the vast majority of the CPUs do not progress up the tree,
+the lock contention remains roughly constant up the tree.
+No matter how many CPUs there are in the system, at most 64 quiescent-state
+reports per grace period will progress all the way to the root
+<tt>rcu_node</tt> structure, thus ensuring that the lock contention
+on that root <tt>rcu_node</tt> structure remains acceptably low.
+
+</p><p>In effect, the combining tree acts like a big shock absorber,
+keeping lock contention under control at all tree levels regardless
+of the level of loading on the system.
+
+</p><p>The Linux kernel actually supports multiple flavors of RCU
+running concurrently, so RCU builds separate data structures for each
+flavor.
+For example, for <tt>CONFIG_TREE_RCU=y</tt> kernels, RCU provides
+rcu_sched and rcu_bh, as shown below:
+
+</p><p><img src="BigTreeClassicRCUBH.svg" alt="BigTreeClassicRCUBH.svg" width="33%">
+
+</p><p>Energy efficiency is increasingly important, and for that
+reason the Linux kernel provides <tt>CONFIG_NO_HZ_IDLE</tt>, which
+turns off the scheduling-clock interrupts on idle CPUs, which in
+turn allows those CPUs to attain deeper sleep states and to consume
+less energy.
+CPUs whose scheduling-clock interrupts have been turned off are
+said to be in <i>dyntick-idle mode</i>.
+RCU must handle dyntick-idle CPUs specially
+because RCU would otherwise wake up each CPU on every grace period,
+which would defeat the whole purpose of <tt>CONFIG_NO_HZ_IDLE</tt>.
+RCU uses the <tt>rcu_dynticks</tt> structure to track
+which CPUs are in dyntick idle mode, as shown below:
+
+</p><p><img src="BigTreeClassicRCUBHdyntick.svg" alt="BigTreeClassicRCUBHdyntick.svg" width="33%">
+
+</p><p>However, if a CPU is in dyntick-idle mode, it is in that mode
+for all flavors of RCU.
+Therefore, a single <tt>rcu_dynticks</tt> structure is allocated per
+CPU, and all of a given CPU's <tt>rcu_data</tt> structures share
+that <tt>rcu_dynticks</tt>, as shown in the figure.
+
+</p><p>Kernels built with <tt>CONFIG_PREEMPT_RCU</tt> support
+rcu_preempt in addition to rcu_sched and rcu_bh, as shown below:
+
+</p><p><img src="BigTreePreemptRCUBHdyntick.svg" alt="BigTreePreemptRCUBHdyntick.svg" width="35%">
+
+</p><p>RCU updaters wait for normal grace periods by registering
+RCU callbacks, either directly via <tt>call_rcu()</tt> and
+friends (namely <tt>call_rcu_bh()</tt> and <tt>call_rcu_sched()</tt>),
+there being a separate interface per flavor of RCU)
+or indirectly via <tt>synchronize_rcu()</tt> and friends.
+RCU callbacks are represented by <tt>rcu_head</tt> structures,
+which are queued on <tt>rcu_data</tt> structures while they are
+waiting for a grace period to elapse, as shown in the following figure:
+
+</p><p><img src="BigTreePreemptRCUBHdyntickCB.svg" alt="BigTreePreemptRCUBHdyntickCB.svg" width="40%">
+
+</p><p>This figure shows how <tt>TREE_RCU</tt>'s and
+<tt>PREEMPT_RCU</tt>'s major data structures are related.
+Lesser data structures will be introduced with the algorithms that
+make use of them.
+
+</p><p>Note that each of the data structures in the above figure has
+its own synchronization:
+
+<p><ol>
+<li>	Each <tt>rcu_state</tt> structures has a lock and a mutex,
+	and some fields are protected by the corresponding root
+	<tt>rcu_node</tt> structure's lock.
+<li>	Each <tt>rcu_node</tt> structure has a spinlock.
+<li>	The fields in <tt>rcu_data</tt> are private to the corresponding
+	CPU, although a few can be read and written by other CPUs.
+<li>	Similarly, the fields in <tt>rcu_dynticks</tt> are private
+	to the corresponding CPU, although a few can be read by
+	other CPUs.
+</ol>
+
+<p>It is important to note that different data structures can have
+very different ideas about the state of RCU at any given time.
+For but one example, awareness of the start or end of a given RCU
+grace period propagates slowly through the data structures.
+This slow propagation is absolutely necessary for RCU to have good
+read-side performance.
+If this balkanized implementation seems foreign to you, one useful
+trick is to consider each instance of these data structures to be
+a different person, each having the usual slightly different
+view of reality.
+
+</p><p>The general role of each of these data structures is as
+follows:
+
+</p><ol>
+<li>	<tt>rcu_state</tt>:
+	This structure forms the interconnection between the
+	<tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
+	tracks grace periods, serves as short-term repository
+	for callbacks orphaned by CPU-hotplug events,
+	maintains <tt>rcu_barrier()</tt> state,
+	tracks expedited grace-period state,
+	and maintains state used to force quiescent states when
+	grace periods extend too long,
+<li>	<tt>rcu_node</tt>: This structure forms the combining
+	tree that propagates quiescent-state
+	information from the leaves to the root, and also propagates
+	grace-period information from the root to the leaves.
+	It provides local copies of the grace-period state in order
+	to allow this information to be accessed in a synchronized
+	manner without suffering the scalability limitations that
+	would otherwise be imposed by global locking.
+	In <tt>CONFIG_PREEMPT_RCU</tt> kernels, it manages the lists
+	of tasks that have blocked while in their current
+	RCU read-side critical section.
+	In <tt>CONFIG_PREEMPT_RCU</tt> with
+	<tt>CONFIG_RCU_BOOST</tt>, it manages the
+	per-<tt>rcu_node</tt> priority-boosting
+	kernel threads (kthreads) and state.
+	Finally, it records CPU-hotplug state in order to determine
+	which CPUs should be ignored during a given grace period.
+<li>	<tt>rcu_data</tt>: This per-CPU structure is the
+	focus of quiescent-state detection and RCU callback queuing.
+	It also tracks its relationship to the corresponding leaf
+	<tt>rcu_node</tt> structure to allow more-efficient
+	propagation of quiescent states up the <tt>rcu_node</tt>
+	combining tree.
+	Like the <tt>rcu_node</tt> structure, it provides a local
+	copy of the grace-period information to allow for-free
+	synchronized
+	access to this information from the corresponding CPU.
+	Finally, this structure records past dyntick-idle state
+	for the corresponding CPU and also tracks statistics.
+<li>	<tt>rcu_dynticks</tt>:
+	This per-CPU structure tracks the current dyntick-idle
+	state for the corresponding CPU.
+	Unlike the other three structures, the <tt>rcu_dynticks</tt>
+	structure is not replicated per RCU flavor.
+<li>	<tt>rcu_head</tt>:
+	This structure represents RCU callbacks, and is the
+	only structure allocated and managed by RCU users.
+	The <tt>rcu_head</tt> structure is normally embedded
+	within the RCU-protected data structure.
+</ol>
+
+<p>If all you wanted from this article was a general notion of how
+RCU's data structures are related, you are done.
+Otherwise, each of the following sections give more details on
+the <tt>rcu_state</tt>, <tt>rcu_node</tt>, <tt>rcu_data</tt>,
+and <tt>rcu_dynticks</tt> data structures.
+
+<h3><a name="The rcu_state Structure">
+The <tt>rcu_state</tt> Structure</a></h3>
+
+<p>The <tt>rcu_state</tt> structure is the base structure that
+represents a flavor of RCU.
+This structure forms the interconnection between the
+<tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
+tracks grace periods, contains the lock used to
+synchronize with CPU-hotplug events,
+and maintains state used to force quiescent states when
+grace periods extend too long,
+
+</p><p>A few of the <tt>rcu_state</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+The more specialized fields are covered in the discussion of their
+use.
+
+<h5>Relationship to rcu_node and rcu_data Structures</h5>
+
+This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+  1   struct rcu_node node[NUM_RCU_NODES];
+  2   struct rcu_node *level[NUM_RCU_LVLS + 1];
+  3   struct rcu_data __percpu *rda;
+</pre>
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Wait a minute!
+	You said that the <tt>rcu_node</tt> structures formed a tree,
+	but they are declared as a flat array!
+	What gives?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	The tree is laid out in the array.
+	The first node In the array is the head, the next set of nodes in the
+	array are children of the head node, and so on until the last set of
+	nodes in the array are the leaves.
+	</font>
+
+	<p><font color="ffffff">See the following diagrams to see how
+	this works.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>The <tt>rcu_node</tt> tree is embedded into the
+<tt>-&gt;node[]</tt> array as shown in the following figure:
+
+</p><p><img src="TreeMapping.svg" alt="TreeMapping.svg" width="40%">
+
+</p><p>One interesting consequence of this mapping is that a
+breadth-first traversal of the tree is implemented as a simple
+linear scan of the array, which is in fact what the
+<tt>rcu_for_each_node_breadth_first()</tt> macro does.
+This macro is used at the beginning and ends of grace periods.
+
+</p><p>Each entry of the <tt>-&gt;level</tt> array references
+the first <tt>rcu_node</tt> structure on the corresponding level
+of the tree, for example, as shown below:
+
+</p><p><img src="TreeMappingLevel.svg" alt="TreeMappingLevel.svg" width="40%">
+
+</p><p>The zero<sup>th</sup> element of the array references the root
+<tt>rcu_node</tt> structure, the first element references the
+first child of the root <tt>rcu_node</tt>, and finally the second
+element references the first leaf <tt>rcu_node</tt> structure.
+
+</p><p>For whatever it is worth, if you draw the tree to be tree-shaped
+rather than array-shaped, it is easy to draw a planar representation:
+
+</p><p><img src="TreeLevel.svg" alt="TreeLevel.svg" width="60%">
+
+</p><p>Finally, the <tt>-&gt;rda</tt> field references a per-CPU
+pointer to the corresponding CPU's <tt>rcu_data</tt> structure.
+
+</p><p>All of these fields are constant once initialization is complete,
+and therefore need no protection.
+
+<h5>Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+  1   unsigned long gp_seq;
+</pre>
+
+<p>RCU grace periods are numbered, and
+the <tt>-&gt;gp_seq</tt> field contains the current grace-period
+sequence number.
+The bottom two bits are the state of the current grace period,
+which can be zero for not yet started or one for in progress.
+In other words, if the bottom two bits of <tt>-&gt;gp_seq</tt> are
+zero, the corresponding flavor of RCU is idle.
+Any other value in the bottom two bits indicates that something is broken.
+This field is protected by the root <tt>rcu_node</tt> structure's
+<tt>-&gt;lock</tt> field.
+
+</p><p>There are <tt>-&gt;gp_seq</tt> fields
+in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures
+as well.
+The fields in the <tt>rcu_state</tt> structure represent the
+most current value, and those of the other structures are compared
+in order to detect the beginnings and ends of grace periods in a distributed
+fashion.
+The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt>
+(down the tree from the root to the leaves) to <tt>rcu_data</tt>.
+
+<h5>Miscellaneous</h5>
+
+<p>This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+  1   unsigned long gp_max;
+  2   char abbr;
+  3   char *name;
+</pre>
+
+<p>The <tt>-&gt;gp_max</tt> field tracks the duration of the longest
+grace period in jiffies.
+It is protected by the root <tt>rcu_node</tt>'s <tt>-&gt;lock</tt>.
+
+<p>The <tt>-&gt;name</tt> field points to the name of the RCU flavor
+(for example, &ldquo;rcu_sched&rdquo;), and is constant.
+The <tt>-&gt;abbr</tt> field contains a one-character abbreviation,
+for example, &ldquo;s&rdquo; for RCU-sched.
+
+<h3><a name="The rcu_node Structure">
+The <tt>rcu_node</tt> Structure</a></h3>
+
+<p>The <tt>rcu_node</tt> structures form the combining
+tree that propagates quiescent-state
+information from the leaves to the root and also that propagates
+grace-period information from the root down to the leaves.
+They provides local copies of the grace-period state in order
+to allow this information to be accessed in a synchronized
+manner without suffering the scalability limitations that
+would otherwise be imposed by global locking.
+In <tt>CONFIG_PREEMPT_RCU</tt> kernels, they manage the lists
+of tasks that have blocked while in their current
+RCU read-side critical section.
+In <tt>CONFIG_PREEMPT_RCU</tt> with
+<tt>CONFIG_RCU_BOOST</tt>, they manage the
+per-<tt>rcu_node</tt> priority-boosting
+kernel threads (kthreads) and state.
+Finally, they record CPU-hotplug state in order to determine
+which CPUs should be ignored during a given grace period.
+
+</p><p>The <tt>rcu_node</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+
+<h5>Connection to Combining Tree</h5>
+
+<p>This portion of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+  1   struct rcu_node *parent;
+  2   u8 level;
+  3   u8 grpnum;
+  4   unsigned long grpmask;
+  5   int grplo;
+  6   int grphi;
+</pre>
+
+<p>The <tt>-&gt;parent</tt> pointer references the <tt>rcu_node</tt>
+one level up in the tree, and is <tt>NULL</tt> for the root
+<tt>rcu_node</tt>.
+The RCU implementation makes heavy use of this field to push quiescent
+states up the tree.
+The <tt>-&gt;level</tt> field gives the level in the tree, with
+the root being at level zero, its children at level one, and so on.
+The <tt>-&gt;grpnum</tt> field gives this node's position within
+the children of its parent, so this number can range between 0 and 31
+on 32-bit systems and between 0 and 63 on 64-bit systems.
+The <tt>-&gt;level</tt> and <tt>-&gt;grpnum</tt> fields are
+used only during initialization and for tracing.
+The <tt>-&gt;grpmask</tt> field is the bitmask counterpart of
+<tt>-&gt;grpnum</tt>, and therefore always has exactly one bit set.
+This mask is used to clear the bit corresponding to this <tt>rcu_node</tt>
+structure in its parent's bitmasks, which are described later.
+Finally, the <tt>-&gt;grplo</tt> and <tt>-&gt;grphi</tt> fields
+contain the lowest and highest numbered CPU served by this
+<tt>rcu_node</tt> structure, respectively.
+
+</p><p>All of these fields are constant, and thus do not require any
+synchronization.
+
+<h5>Synchronization</h5>
+
+<p>This field of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+  1   raw_spinlock_t lock;
+</pre>
+
+<p>This field is used to protect the remaining fields in this structure,
+unless otherwise stated.
+That said, all of the fields in this structure can be accessed without
+locking for tracing purposes.
+Yes, this can result in confusing traces, but better some tracing confusion
+than to be heisenbugged out of existence.
+
+<h5>Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+  1   unsigned long gp_seq;
+  2   unsigned long gp_seq_needed;
+</pre>
+
+<p>The <tt>rcu_node</tt> structures' <tt>-&gt;gp_seq</tt> fields are
+the counterparts of the field of the same name in the <tt>rcu_state</tt>
+structure.
+They each may lag up to one step behind their <tt>rcu_state</tt>
+counterpart.
+If the bottom two bits of a given <tt>rcu_node</tt> structure's
+<tt>-&gt;gp_seq</tt> field is zero, then this <tt>rcu_node</tt>
+structure believes that RCU is idle.
+</p><p>The <tt>&gt;gp_seq</tt> field of each <tt>rcu_node</tt>
+structure is updated at the beginning and the end
+of each grace period.
+
+<p>The <tt>-&gt;gp_seq_needed</tt> fields record the
+furthest-in-the-future grace period request seen by the corresponding
+<tt>rcu_node</tt> structure.  The request is considered fulfilled when
+the value of the <tt>-&gt;gp_seq</tt> field equals or exceeds that of
+the <tt>-&gt;gp_seq_needed</tt> field.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Suppose that this <tt>rcu_node</tt> structure doesn't see
+	a request for a very long time.
+	Won't wrapping of the <tt>-&gt;gp_seq</tt> field cause
+	problems?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	No, because if the <tt>-&gt;gp_seq_needed</tt> field lags behind the
+	<tt>-&gt;gp_seq</tt> field, the <tt>-&gt;gp_seq_needed</tt> field
+	will be updated at the end of the grace period.
+	Modulo-arithmetic comparisons therefore will always get the
+	correct answer, even with wrapping.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h5>Quiescent-State Tracking</h5>
+
+<p>These fields manage the propagation of quiescent states up the
+combining tree.
+
+</p><p>This portion of the <tt>rcu_node</tt> structure has fields
+as follows:
+
+<pre>
+  1   unsigned long qsmask;
+  2   unsigned long expmask;
+  3   unsigned long qsmaskinit;
+  4   unsigned long expmaskinit;
+</pre>
+
+<p>The <tt>-&gt;qsmask</tt> field tracks which of this
+<tt>rcu_node</tt> structure's children still need to report
+quiescent states for the current normal grace period.
+Such children will have a value of 1 in their corresponding bit.
+Note that the leaf <tt>rcu_node</tt> structures should be
+thought of as having <tt>rcu_data</tt> structures as their
+children.
+Similarly, the <tt>-&gt;expmask</tt> field tracks which
+of this <tt>rcu_node</tt> structure's children still need to report
+quiescent states for the current expedited grace period.
+An expedited grace period has
+the same conceptual properties as a normal grace period, but the
+expedited implementation accepts extreme CPU overhead to obtain
+much lower grace-period latency, for example, consuming a few
+tens of microseconds worth of CPU time to reduce grace-period
+duration from milliseconds to tens of microseconds.
+The <tt>-&gt;qsmaskinit</tt> field tracks which of this
+<tt>rcu_node</tt> structure's children cover for at least
+one online CPU.
+This mask is used to initialize <tt>-&gt;qsmask</tt>,
+and <tt>-&gt;expmaskinit</tt> is used to initialize
+<tt>-&gt;expmask</tt> and the beginning of the
+normal and expedited grace periods, respectively.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why are these bitmasks protected by locking?
+	Come on, haven't you heard of atomic instructions???
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Lockless grace-period computation!  Such a tantalizing possibility!
+	</font>
+
+	<p><font color="ffffff">But consider the following sequence of events:
+	</font>
+
+	<ol>
+	<li>	<font color="ffffff">CPU&nbsp;0 has been in dyntick-idle
+		mode for quite some time.
+		When it wakes up, it notices that the current RCU
+		grace period needs it to report in, so it sets a
+		flag where the scheduling clock interrupt will find it.
+		</font><p>
+	<li>	<font color="ffffff">Meanwhile, CPU&nbsp;1 is running
+		<tt>force_quiescent_state()</tt>,
+		and notices that CPU&nbsp;0 has been in dyntick idle mode,
+		which qualifies as an extended quiescent state.
+		</font><p>
+	<li>	<font color="ffffff">CPU&nbsp;0's scheduling clock
+		interrupt fires in the
+		middle of an RCU read-side critical section, and notices
+		that the RCU core needs something, so commences RCU softirq
+		processing.
+		</font>
+		<p>
+	<li>	<font color="ffffff">CPU&nbsp;0's softirq handler
+		executes and is just about ready
+		to report its quiescent state up the <tt>rcu_node</tt>
+		tree.
+		</font><p>
+	<li>	<font color="ffffff">But CPU&nbsp;1 beats it to the punch,
+		completing the current
+		grace period and starting a new one.
+		</font><p>
+	<li>	<font color="ffffff">CPU&nbsp;0 now reports its quiescent
+		state for the wrong
+		grace period.
+		That grace period might now end before the RCU read-side
+		critical section.
+		If that happens, disaster will ensue.
+		</font>
+	</ol>
+
+	<p><font color="ffffff">So the locking is absolutely required in
+	order to coordinate clearing of the bits with updating of the
+	grace-period sequence number in <tt>-&gt;gp_seq</tt>.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h5>Blocked-Task Management</h5>
+
+<p><tt>PREEMPT_RCU</tt> allows tasks to be preempted in the
+midst of their RCU read-side critical sections, and these tasks
+must be tracked explicitly.
+The details of exactly why and how they are tracked will be covered
+in a separate article on RCU read-side processing.
+For now, it is enough to know that the <tt>rcu_node</tt>
+structure tracks them.
+
+<pre>
+  1   struct list_head blkd_tasks;
+  2   struct list_head *gp_tasks;
+  3   struct list_head *exp_tasks;
+  4   bool wait_blkd_tasks;
+</pre>
+
+<p>The <tt>-&gt;blkd_tasks</tt> field is a list header for
+the list of blocked and preempted tasks.
+As tasks undergo context switches within RCU read-side critical
+sections, their <tt>task_struct</tt> structures are enqueued
+(via the <tt>task_struct</tt>'s <tt>-&gt;rcu_node_entry</tt>
+field) onto the head of the <tt>-&gt;blkd_tasks</tt> list for the
+leaf <tt>rcu_node</tt> structure corresponding to the CPU
+on which the outgoing context switch executed.
+As these tasks later exit their RCU read-side critical sections,
+they remove themselves from the list.
+This list is therefore in reverse time order, so that if one of the tasks
+is blocking the current grace period, all subsequent tasks must
+also be blocking that same grace period.
+Therefore, a single pointer into this list suffices to track
+all tasks blocking a given grace period.
+That pointer is stored in <tt>-&gt;gp_tasks</tt> for normal
+grace periods and in <tt>-&gt;exp_tasks</tt> for expedited
+grace periods.
+These last two fields are <tt>NULL</tt> if either there is
+no grace period in flight or if there are no blocked tasks
+preventing that grace period from completing.
+If either of these two pointers is referencing a task that
+removes itself from the <tt>-&gt;blkd_tasks</tt> list,
+then that task must advance the pointer to the next task on
+the list, or set the pointer to <tt>NULL</tt> if there
+are no subsequent tasks on the list.
+
+</p><p>For example, suppose that tasks&nbsp;T1, T2, and&nbsp;T3 are
+all hard-affinitied to the largest-numbered CPU in the system.
+Then if task&nbsp;T1 blocked in an RCU read-side
+critical section, then an expedited grace period started,
+then task&nbsp;T2 blocked in an RCU read-side critical section,
+then a normal grace period started, and finally task&nbsp;3 blocked
+in an RCU read-side critical section, then the state of the
+last leaf <tt>rcu_node</tt> structure's blocked-task list
+would be as shown below:
+
+</p><p><img src="blkd_task.svg" alt="blkd_task.svg" width="60%">
+
+</p><p>Task&nbsp;T1 is blocking both grace periods, task&nbsp;T2 is
+blocking only the normal grace period, and task&nbsp;T3 is blocking
+neither grace period.
+Note that these tasks will not remove themselves from this list
+immediately upon resuming execution.
+They will instead remain on the list until they execute the outermost
+<tt>rcu_read_unlock()</tt> that ends their RCU read-side critical
+section.
+
+<p>
+The <tt>-&gt;wait_blkd_tasks</tt> field indicates whether or not
+the current grace period is waiting on a blocked task.
+
+<h5>Sizing the <tt>rcu_node</tt> Array</h5>
+
+<p>The <tt>rcu_node</tt> array is sized via a series of
+C-preprocessor expressions as follows:
+
+<pre>
+ 1 #ifdef CONFIG_RCU_FANOUT
+ 2 #define RCU_FANOUT CONFIG_RCU_FANOUT
+ 3 #else
+ 4 # ifdef CONFIG_64BIT
+ 5 # define RCU_FANOUT 64
+ 6 # else
+ 7 # define RCU_FANOUT 32
+ 8 # endif
+ 9 #endif
+10
+11 #ifdef CONFIG_RCU_FANOUT_LEAF
+12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
+13 #else
+14 # ifdef CONFIG_64BIT
+15 # define RCU_FANOUT_LEAF 64
+16 # else
+17 # define RCU_FANOUT_LEAF 32
+18 # endif
+19 #endif
+20
+21 #define RCU_FANOUT_1        (RCU_FANOUT_LEAF)
+22 #define RCU_FANOUT_2        (RCU_FANOUT_1 * RCU_FANOUT)
+23 #define RCU_FANOUT_3        (RCU_FANOUT_2 * RCU_FANOUT)
+24 #define RCU_FANOUT_4        (RCU_FANOUT_3 * RCU_FANOUT)
+25
+26 #if NR_CPUS &lt;= RCU_FANOUT_1
+27 #  define RCU_NUM_LVLS        1
+28 #  define NUM_RCU_LVL_0        1
+29 #  define NUM_RCU_NODES        NUM_RCU_LVL_0
+30 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0 }
+31 #  define RCU_NODE_NAME_INIT  { "rcu_node_0" }
+32 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0" }
+33 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0" }
+34 #elif NR_CPUS &lt;= RCU_FANOUT_2
+35 #  define RCU_NUM_LVLS        2
+36 #  define NUM_RCU_LVL_0        1
+37 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+38 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1)
+39 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
+40 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1" }
+41 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1" }
+42 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1" }
+43 #elif NR_CPUS &lt;= RCU_FANOUT_3
+44 #  define RCU_NUM_LVLS        3
+45 #  define NUM_RCU_LVL_0        1
+46 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
+47 #  define NUM_RCU_LVL_2        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+48 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2)
+49 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
+50 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
+51 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
+52 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" }
+53 #elif NR_CPUS &lt;= RCU_FANOUT_4
+54 #  define RCU_NUM_LVLS        4
+55 #  define NUM_RCU_LVL_0        1
+56 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3)
+57 #  define NUM_RCU_LVL_2        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
+58 #  define NUM_RCU_LVL_3        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+59 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
+60 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
+61 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
+62 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
+63 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" }
+64 #else
+65 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
+66 #endif
+</pre>
+
+<p>The maximum number of levels in the <tt>rcu_node</tt> structure
+is currently limited to four, as specified by lines&nbsp;21-24
+and the structure of the subsequent &ldquo;if&rdquo; statement.
+For 32-bit systems, this allows 16*32*32*32=524,288 CPUs, which
+should be sufficient for the next few years at least.
+For 64-bit systems, 16*64*64*64=4,194,304 CPUs is allowed, which
+should see us through the next decade or so.
+This four-level tree also allows kernels built with
+<tt>CONFIG_RCU_FANOUT=8</tt> to support up to 4096 CPUs,
+which might be useful in very large systems having eight CPUs per
+socket (but please note that no one has yet shown any measurable
+performance degradation due to misaligned socket and <tt>rcu_node</tt>
+boundaries).
+In addition, building kernels with a full four levels of <tt>rcu_node</tt>
+tree permits better testing of RCU's combining-tree code.
+
+</p><p>The <tt>RCU_FANOUT</tt> symbol controls how many children
+are permitted at each non-leaf level of the <tt>rcu_node</tt> tree.
+If the <tt>CONFIG_RCU_FANOUT</tt> Kconfig option is not specified,
+it is set based on the word size of the system, which is also
+the Kconfig default.
+
+</p><p>The <tt>RCU_FANOUT_LEAF</tt> symbol controls how many CPUs are
+handled by each leaf <tt>rcu_node</tt> structure.
+Experience has shown that allowing a given leaf <tt>rcu_node</tt>
+structure to handle 64 CPUs, as permitted by the number of bits in
+the <tt>-&gt;qsmask</tt> field on a 64-bit system, results in
+excessive contention for the leaf <tt>rcu_node</tt> structures'
+<tt>-&gt;lock</tt> fields.
+The number of CPUs per leaf <tt>rcu_node</tt> structure is therefore
+limited to 16 given the default value of <tt>CONFIG_RCU_FANOUT_LEAF</tt>.
+If <tt>CONFIG_RCU_FANOUT_LEAF</tt> is unspecified, the value
+selected is based on the word size of the system, just as for
+<tt>CONFIG_RCU_FANOUT</tt>.
+Lines&nbsp;11-19 perform this computation.
+
+</p><p>Lines&nbsp;21-24 compute the maximum number of CPUs supported by
+a single-level (which contains a single <tt>rcu_node</tt> structure),
+two-level, three-level, and four-level <tt>rcu_node</tt> tree,
+respectively, given the fanout specified by <tt>RCU_FANOUT</tt>
+and <tt>RCU_FANOUT_LEAF</tt>.
+These numbers of CPUs are retained in the
+<tt>RCU_FANOUT_1</tt>,
+<tt>RCU_FANOUT_2</tt>,
+<tt>RCU_FANOUT_3</tt>, and
+<tt>RCU_FANOUT_4</tt>
+C-preprocessor variables, respectively.
+
+</p><p>These variables are used to control the C-preprocessor <tt>#if</tt>
+statement spanning lines&nbsp;26-66 that computes the number of
+<tt>rcu_node</tt> structures required for each level of the tree,
+as well as the number of levels required.
+The number of levels is placed in the <tt>NUM_RCU_LVLS</tt>
+C-preprocessor variable by lines&nbsp;27, 35, 44, and&nbsp;54.
+The number of <tt>rcu_node</tt> structures for the topmost level
+of the tree is always exactly one, and this value is unconditionally
+placed into <tt>NUM_RCU_LVL_0</tt> by lines&nbsp;28, 36, 45, and&nbsp;55.
+The rest of the levels (if any) of the <tt>rcu_node</tt> tree
+are computed by dividing the maximum number of CPUs by the
+fanout supported by the number of levels from the current level down,
+rounding up.  This computation is performed by lines&nbsp;37,
+46-47, and&nbsp;56-58.
+Lines&nbsp;31-33, 40-42, 50-52, and&nbsp;62-63 create initializers
+for lockdep lock-class names.
+Finally, lines&nbsp;64-66 produce an error if the maximum number of
+CPUs is too large for the specified fanout.
+
+<h3><a name="The rcu_segcblist Structure">
+The <tt>rcu_segcblist</tt> Structure</a></h3>
+
+The <tt>rcu_segcblist</tt> structure maintains a segmented list of
+callbacks as follows:
+
+<pre>
+ 1 #define RCU_DONE_TAIL        0
+ 2 #define RCU_WAIT_TAIL        1
+ 3 #define RCU_NEXT_READY_TAIL  2
+ 4 #define RCU_NEXT_TAIL        3
+ 5 #define RCU_CBLIST_NSEGS     4
+ 6
+ 7 struct rcu_segcblist {
+ 8   struct rcu_head *head;
+ 9   struct rcu_head **tails[RCU_CBLIST_NSEGS];
+10   unsigned long gp_seq[RCU_CBLIST_NSEGS];
+11   long len;
+12   long len_lazy;
+13 };
+</pre>
+
+<p>
+The segments are as follows:
+
+<ol>
+<li>	<tt>RCU_DONE_TAIL</tt>: Callbacks whose grace periods have elapsed.
+	These callbacks are ready to be invoked.
+<li>	<tt>RCU_WAIT_TAIL</tt>: Callbacks that are waiting for the
+	current grace period.
+	Note that different CPUs can have different ideas about which
+	grace period is current, hence the <tt>-&gt;gp_seq</tt> field.
+<li>	<tt>RCU_NEXT_READY_TAIL</tt>: Callbacks waiting for the next
+	grace period to start.
+<li>	<tt>RCU_NEXT_TAIL</tt>: Callbacks that have not yet been
+	associated with a grace period.
+</ol>
+
+<p>
+The <tt>-&gt;head</tt> pointer references the first callback or
+is <tt>NULL</tt> if the list contains no callbacks (which is
+<i>not</i> the same as being empty).
+Each element of the <tt>-&gt;tails[]</tt> array references the
+<tt>-&gt;next</tt> pointer of the last callback in the corresponding
+segment of the list, or the list's <tt>-&gt;head</tt> pointer if
+that segment and all previous segments are empty.
+If the corresponding segment is empty but some previous segment is
+not empty, then the array element is identical to its predecessor.
+Older callbacks are closer to the head of the list, and new callbacks
+are added at the tail.
+This relationship between the <tt>-&gt;head</tt> pointer, the
+<tt>-&gt;tails[]</tt> array, and the callbacks is shown in this
+diagram:
+
+</p><p><img src="nxtlist.svg" alt="nxtlist.svg" width="40%">
+
+</p><p>In this figure, the <tt>-&gt;head</tt> pointer references the
+first
+RCU callback in the list.
+The <tt>-&gt;tails[RCU_DONE_TAIL]</tt> array element references
+the <tt>-&gt;head</tt> pointer itself, indicating that none
+of the callbacks is ready to invoke.
+The <tt>-&gt;tails[RCU_WAIT_TAIL]</tt> array element references callback
+CB&nbsp;2's <tt>-&gt;next</tt> pointer, which indicates that
+CB&nbsp;1 and CB&nbsp;2 are both waiting on the current grace period,
+give or take possible disagreements about exactly which grace period
+is the current one.
+The <tt>-&gt;tails[RCU_NEXT_READY_TAIL]</tt> array element
+references the same RCU callback that <tt>-&gt;tails[RCU_WAIT_TAIL]</tt>
+does, which indicates that there are no callbacks waiting on the next
+RCU grace period.
+The <tt>-&gt;tails[RCU_NEXT_TAIL]</tt> array element references
+CB&nbsp;4's <tt>-&gt;next</tt> pointer, indicating that all the
+remaining RCU callbacks have not yet been assigned to an RCU grace
+period.
+Note that the <tt>-&gt;tails[RCU_NEXT_TAIL]</tt> array element
+always references the last RCU callback's <tt>-&gt;next</tt> pointer
+unless the callback list is empty, in which case it references
+the <tt>-&gt;head</tt> pointer.
+
+<p>
+There is one additional important special case for the
+<tt>-&gt;tails[RCU_NEXT_TAIL]</tt> array element: It can be <tt>NULL</tt>
+when this list is <i>disabled</i>.
+Lists are disabled when the corresponding CPU is offline or when
+the corresponding CPU's callbacks are offloaded to a kthread,
+both of which are described elsewhere.
+
+</p><p>CPUs advance their callbacks from the
+<tt>RCU_NEXT_TAIL</tt> to the <tt>RCU_NEXT_READY_TAIL</tt> to the
+<tt>RCU_WAIT_TAIL</tt> to the <tt>RCU_DONE_TAIL</tt> list segments
+as grace periods advance.
+
+</p><p>The <tt>-&gt;gp_seq[]</tt> array records grace-period
+numbers corresponding to the list segments.
+This is what allows different CPUs to have different ideas as to
+which is the current grace period while still avoiding premature
+invocation of their callbacks.
+In particular, this allows CPUs that go idle for extended periods
+to determine which of their callbacks are ready to be invoked after
+reawakening.
+
+</p><p>The <tt>-&gt;len</tt> counter contains the number of
+callbacks in <tt>-&gt;head</tt>, and the
+<tt>-&gt;len_lazy</tt> contains the number of those callbacks that
+are known to only free memory, and whose invocation can therefore
+be safely deferred.
+
+<p><b>Important note</b>: It is the <tt>-&gt;len</tt> field that
+determines whether or not there are callbacks associated with
+this <tt>rcu_segcblist</tt> structure, <i>not</i> the <tt>-&gt;head</tt>
+pointer.
+The reason for this is that all the ready-to-invoke callbacks
+(that is, those in the <tt>RCU_DONE_TAIL</tt> segment) are extracted
+all at once at callback-invocation time.
+If callback invocation must be postponed, for example, because a
+high-priority process just woke up on this CPU, then the remaining
+callbacks are placed back on the <tt>RCU_DONE_TAIL</tt> segment.
+Either way, the <tt>-&gt;len</tt> and <tt>-&gt;len_lazy</tt> counts
+are adjusted after the corresponding callbacks have been invoked, and so
+again it is the <tt>-&gt;len</tt> count that accurately reflects whether
+or not there are callbacks associated with this <tt>rcu_segcblist</tt>
+structure.
+Of course, off-CPU sampling of the <tt>-&gt;len</tt> count requires
+the use of appropriate synchronization, for example, memory barriers.
+This synchronization can be a bit subtle, particularly in the case
+of <tt>rcu_barrier()</tt>.
+
+<h3><a name="The rcu_data Structure">
+The <tt>rcu_data</tt> Structure</a></h3>
+
+<p>The <tt>rcu_data</tt> maintains the per-CPU state for the
+corresponding flavor of RCU.
+The fields in this structure may be accessed only from the corresponding
+CPU (and from tracing) unless otherwise stated.
+This structure is the
+focus of quiescent-state detection and RCU callback queuing.
+It also tracks its relationship to the corresponding leaf
+<tt>rcu_node</tt> structure to allow more-efficient
+propagation of quiescent states up the <tt>rcu_node</tt>
+combining tree.
+Like the <tt>rcu_node</tt> structure, it provides a local
+copy of the grace-period information to allow for-free
+synchronized
+access to this information from the corresponding CPU.
+Finally, this structure records past dyntick-idle state
+for the corresponding CPU and also tracks statistics.
+
+</p><p>The <tt>rcu_data</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+
+<h5>Connection to Other Data Structures</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+  1   int cpu;
+  2   struct rcu_state *rsp;
+  3   struct rcu_node *mynode;
+  4   struct rcu_dynticks *dynticks;
+  5   unsigned long grpmask;
+  6   bool beenonline;
+</pre>
+
+<p>The <tt>-&gt;cpu</tt> field contains the number of the
+corresponding CPU, the <tt>-&gt;rsp</tt> pointer references
+the corresponding <tt>rcu_state</tt> structure (and is most frequently
+used to locate the name of the corresponding flavor of RCU for tracing),
+and the <tt>-&gt;mynode</tt> field references the corresponding
+<tt>rcu_node</tt> structure.
+The <tt>-&gt;mynode</tt> is used to propagate quiescent states
+up the combining tree.
+<p>The <tt>-&gt;dynticks</tt> pointer references the
+<tt>rcu_dynticks</tt> structure corresponding to this
+CPU.
+Recall that a single per-CPU instance of the <tt>rcu_dynticks</tt>
+structure is shared among all flavors of RCU.
+These first four fields are constant and therefore require not
+synchronization.
+
+</p><p>The <tt>-&gt;grpmask</tt> field indicates the bit in
+the <tt>-&gt;mynode-&gt;qsmask</tt> corresponding to this
+<tt>rcu_data</tt> structure, and is also used when propagating
+quiescent states.
+The <tt>-&gt;beenonline</tt> flag is set whenever the corresponding
+CPU comes online, which means that the debugfs tracing need not dump
+out any <tt>rcu_data</tt> structure for which this flag is not set.
+
+<h5>Quiescent-State and Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+  1   unsigned long gp_seq;
+  2   unsigned long gp_seq_needed;
+  3   bool cpu_no_qs;
+  4   bool core_needs_qs;
+  5   bool gpwrap;
+  6   unsigned long rcu_qs_ctr_snap;
+</pre>
+
+<p>The <tt>-&gt;gp_seq</tt> and <tt>-&gt;gp_seq_needed</tt>
+fields are the counterparts of the fields of the same name
+in the <tt>rcu_state</tt> and <tt>rcu_node</tt> structures.
+They may each lag up to one behind their <tt>rcu_node</tt>
+counterparts, but in <tt>CONFIG_NO_HZ_IDLE</tt> and
+<tt>CONFIG_NO_HZ_FULL</tt> kernels can lag
+arbitrarily far behind for CPUs in dyntick-idle mode (but these counters
+will catch up upon exit from dyntick-idle mode).
+If the lower two bits of a given <tt>rcu_data</tt> structure's
+<tt>-&gt;gp_seq</tt> are zero, then this <tt>rcu_data</tt>
+structure believes that RCU is idle.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	All this replication of the grace period numbers can only cause
+	massive confusion.
+	Why not just keep a global sequence number and be done with it???
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Because if there was only a single global sequence
+	numbers, there would need to be a single global lock to allow
+	safely accessing and updating it.
+	And if we are not going to have a single global lock, we need
+	to carefully manage the numbers on a per-node basis.
+	Recall from the answer to a previous Quick Quiz that the consequences
+	of applying a previously sampled quiescent state to the wrong
+	grace period are quite severe.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>The <tt>-&gt;cpu_no_qs</tt> flag indicates that the
+CPU has not yet passed through a quiescent state,
+while the <tt>-&gt;core_needs_qs</tt> flag indicates that the
+RCU core needs a quiescent state from the corresponding CPU.
+The <tt>-&gt;gpwrap</tt> field indicates that the corresponding
+CPU has remained idle for so long that the
+<tt>gp_seq</tt> counter is in danger of overflow, which
+will cause the CPU to disregard the values of its counters on
+its next exit from idle.
+Finally, the <tt>rcu_qs_ctr_snap</tt> field is used to detect
+cases where a given operation has resulted in a quiescent state
+for all flavors of RCU, for example, <tt>cond_resched()</tt>
+when RCU has indicated a need for quiescent states.
+
+<h5>RCU Callback Handling</h5>
+
+<p>In the absence of CPU-hotplug events, RCU callbacks are invoked by
+the same CPU that registered them.
+This is strictly a cache-locality optimization: callbacks can and
+do get invoked on CPUs other than the one that registered them.
+After all, if the CPU that registered a given callback has gone
+offline before the callback can be invoked, there really is no other
+choice.
+
+</p><p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+ 1 struct rcu_segcblist cblist;
+ 2 long qlen_last_fqs_check;
+ 3 unsigned long n_cbs_invoked;
+ 4 unsigned long n_nocbs_invoked;
+ 5 unsigned long n_cbs_orphaned;
+ 6 unsigned long n_cbs_adopted;
+ 7 unsigned long n_force_qs_snap;
+ 8 long blimit;
+</pre>
+
+<p>The <tt>-&gt;cblist</tt> structure is the segmented callback list
+described earlier.
+The CPU advances the callbacks in its <tt>rcu_data</tt> structure
+whenever it notices that another RCU grace period has completed.
+The CPU detects the completion of an RCU grace period by noticing
+that the value of its <tt>rcu_data</tt> structure's
+<tt>-&gt;gp_seq</tt> field differs from that of its leaf
+<tt>rcu_node</tt> structure.
+Recall that each <tt>rcu_node</tt> structure's
+<tt>-&gt;gp_seq</tt> field is updated at the beginnings and ends of each
+grace period.
+
+<p>
+The <tt>-&gt;qlen_last_fqs_check</tt> and
+<tt>-&gt;n_force_qs_snap</tt> coordinate the forcing of quiescent
+states from <tt>call_rcu()</tt> and friends when callback
+lists grow excessively long.
+
+</p><p>The <tt>-&gt;n_cbs_invoked</tt>,
+<tt>-&gt;n_cbs_orphaned</tt>, and <tt>-&gt;n_cbs_adopted</tt>
+fields count the number of callbacks invoked,
+sent to other CPUs when this CPU goes offline,
+and received from other CPUs when those other CPUs go offline.
+The <tt>-&gt;n_nocbs_invoked</tt> is used when the CPU's callbacks
+are offloaded to a kthread.
+
+<p>
+Finally, the <tt>-&gt;blimit</tt> counter is the maximum number of
+RCU callbacks that may be invoked at a given time.
+
+<h5>Dyntick-Idle Handling</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+  1   int dynticks_snap;
+  2   unsigned long dynticks_fqs;
+</pre>
+
+The <tt>-&gt;dynticks_snap</tt> field is used to take a snapshot
+of the corresponding CPU's dyntick-idle state when forcing
+quiescent states, and is therefore accessed from other CPUs.
+Finally, the <tt>-&gt;dynticks_fqs</tt> field is used to
+count the number of times this CPU is determined to be in
+dyntick-idle state, and is used for tracing and debugging purposes.
+
+<h3><a name="The rcu_dynticks Structure">
+The <tt>rcu_dynticks</tt> Structure</a></h3>
+
+<p>The <tt>rcu_dynticks</tt> maintains the per-CPU dyntick-idle state
+for the corresponding CPU.
+Unlike the other structures, <tt>rcu_dynticks</tt> is not
+replicated over the different flavors of RCU.
+The fields in this structure may be accessed only from the corresponding
+CPU (and from tracing) unless otherwise stated.
+Its fields are as follows:
+
+<pre>
+  1   long dynticks_nesting;
+  2   long dynticks_nmi_nesting;
+  3   atomic_t dynticks;
+  4   bool rcu_need_heavy_qs;
+  5   unsigned long rcu_qs_ctr;
+  6   bool rcu_urgent_qs;
+</pre>
+
+<p>The <tt>-&gt;dynticks_nesting</tt> field counts the
+nesting depth of process execution, so that in normal circumstances
+this counter has value zero or one.
+NMIs, irqs, and tracers are counted by the <tt>-&gt;dynticks_nmi_nesting</tt>
+field.
+Because NMIs cannot be masked, changes to this variable have to be
+undertaken carefully using an algorithm provided by Andy Lutomirski.
+The initial transition from idle adds one, and nested transitions
+add two, so that a nesting level of five is represented by a
+<tt>-&gt;dynticks_nmi_nesting</tt> value of nine.
+This counter can therefore be thought of as counting the number
+of reasons why this CPU cannot be permitted to enter dyntick-idle
+mode, aside from process-level transitions.
+
+<p>However, it turns out that when running in non-idle kernel context,
+the Linux kernel is fully capable of entering interrupt handlers that
+never exit and perhaps also vice versa.
+Therefore, whenever the <tt>-&gt;dynticks_nesting</tt> field is
+incremented up from zero, the <tt>-&gt;dynticks_nmi_nesting</tt> field
+is set to a large positive number, and whenever the
+<tt>-&gt;dynticks_nesting</tt> field is decremented down to zero,
+the the <tt>-&gt;dynticks_nmi_nesting</tt> field is set to zero.
+Assuming that the number of misnested interrupts is not sufficient
+to overflow the counter, this approach corrects the
+<tt>-&gt;dynticks_nmi_nesting</tt> field every time the corresponding
+CPU enters the idle loop from process context.
+
+</p><p>The <tt>-&gt;dynticks</tt> field counts the corresponding
+CPU's transitions to and from dyntick-idle mode, so that this counter
+has an even value when the CPU is in dyntick-idle mode and an odd
+value otherwise.
+
+</p><p>The <tt>-&gt;rcu_need_heavy_qs</tt> field is used
+to record the fact that the RCU core code would really like to
+see a quiescent state from the corresponding CPU, so much so that
+it is willing to call for heavy-weight dyntick-counter operations.
+This flag is checked by RCU's context-switch and <tt>cond_resched()</tt>
+code, which provide a momentary idle sojourn in response.
+
+</p><p>The <tt>-&gt;rcu_qs_ctr</tt> field is used to record
+quiescent states from <tt>cond_resched()</tt>.
+Because <tt>cond_resched()</tt> can execute quite frequently, this
+must be quite lightweight, as in a non-atomic increment of this
+per-CPU field.
+
+</p><p>Finally, the <tt>-&gt;rcu_urgent_qs</tt> field is used to record
+the fact that the RCU core code would really like to see a quiescent
+state from the corresponding CPU, with the various other fields indicating
+just how badly RCU wants this quiescent state.
+This flag is checked by RCU's context-switch and <tt>cond_resched()</tt>
+code, which, if nothing else, non-atomically increment <tt>-&gt;rcu_qs_ctr</tt>
+in response.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why not simply combine the <tt>-&gt;dynticks_nesting</tt>
+	and <tt>-&gt;dynticks_nmi_nesting</tt> counters into a
+	single counter that just counts the number of reasons that
+	the corresponding CPU is non-idle?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Because this would fail in the presence of interrupts whose
+	handlers never return and of handlers that manage to return
+	from a made-up interrupt.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>Additional fields are present for some special-purpose
+builds, and are discussed separately.
+
+<h3><a name="The rcu_head Structure">
+The <tt>rcu_head</tt> Structure</a></h3>
+
+<p>Each <tt>rcu_head</tt> structure represents an RCU callback.
+These structures are normally embedded within RCU-protected data
+structures whose algorithms use asynchronous grace periods.
+In contrast, when using algorithms that block waiting for RCU grace periods,
+RCU users need not provide <tt>rcu_head</tt> structures.
+
+</p><p>The <tt>rcu_head</tt> structure has fields as follows:
+
+<pre>
+  1   struct rcu_head *next;
+  2   void (*func)(struct rcu_head *head);
+</pre>
+
+<p>The <tt>-&gt;next</tt> field is used
+to link the <tt>rcu_head</tt> structures together in the
+lists within the <tt>rcu_data</tt> structures.
+The <tt>-&gt;func</tt> field is a pointer to the function
+to be called when the callback is ready to be invoked, and
+this function is passed a pointer to the <tt>rcu_head</tt>
+structure.
+However, <tt>kfree_rcu()</tt> uses the <tt>-&gt;func</tt>
+field to record the offset of the <tt>rcu_head</tt>
+structure within the enclosing RCU-protected data structure.
+
+</p><p>Both of these fields are used internally by RCU.
+From the viewpoint of RCU users, this structure is an
+opaque &ldquo;cookie&rdquo;.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Given that the callback function <tt>-&gt;func</tt>
+	is passed a pointer to the <tt>rcu_head</tt> structure,
+	how is that function supposed to find the beginning of the
+	enclosing RCU-protected data structure?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	In actual practice, there is a separate callback function per
+	type of RCU-protected data structure.
+	The callback function can therefore use the <tt>container_of()</tt>
+	macro in the Linux kernel (or other pointer-manipulation facilities
+	in other software environments) to find the beginning of the
+	enclosing structure.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h3><a name="RCU-Specific Fields in the task_struct Structure">
+RCU-Specific Fields in the <tt>task_struct</tt> Structure</a></h3>
+
+<p>The <tt>CONFIG_PREEMPT_RCU</tt> implementation uses some
+additional fields in the <tt>task_struct</tt> structure:
+
+<pre>
+ 1 #ifdef CONFIG_PREEMPT_RCU
+ 2   int rcu_read_lock_nesting;
+ 3   union rcu_special rcu_read_unlock_special;
+ 4   struct list_head rcu_node_entry;
+ 5   struct rcu_node *rcu_blocked_node;
+ 6 #endif /* #ifdef CONFIG_PREEMPT_RCU */
+ 7 #ifdef CONFIG_TASKS_RCU
+ 8   unsigned long rcu_tasks_nvcsw;
+ 9   bool rcu_tasks_holdout;
+10   struct list_head rcu_tasks_holdout_list;
+11   int rcu_tasks_idle_cpu;
+12 #endif /* #ifdef CONFIG_TASKS_RCU */
+</pre>
+
+<p>The <tt>-&gt;rcu_read_lock_nesting</tt> field records the
+nesting level for RCU read-side critical sections, and
+the <tt>-&gt;rcu_read_unlock_special</tt> field is a bitmask
+that records special conditions that require <tt>rcu_read_unlock()</tt>
+to do additional work.
+The <tt>-&gt;rcu_node_entry</tt> field is used to form lists of
+tasks that have blocked within preemptible-RCU read-side critical
+sections and the <tt>-&gt;rcu_blocked_node</tt> field references
+the <tt>rcu_node</tt> structure whose list this task is a member of,
+or <tt>NULL</tt> if it is not blocked within a preemptible-RCU
+read-side critical section.
+
+<p>The <tt>-&gt;rcu_tasks_nvcsw</tt> field tracks the number of
+voluntary context switches that this task had undergone at the
+beginning of the current tasks-RCU grace period,
+<tt>-&gt;rcu_tasks_holdout</tt> is set if the current tasks-RCU
+grace period is waiting on this task, <tt>-&gt;rcu_tasks_holdout_list</tt>
+is a list element enqueuing this task on the holdout list,
+and <tt>-&gt;rcu_tasks_idle_cpu</tt> tracks which CPU this
+idle task is running, but only if the task is currently running,
+that is, if the CPU is currently idle.
+
+<h3><a name="Accessor Functions">
+Accessor Functions</a></h3>
+
+<p>The following listing shows the
+<tt>rcu_get_root()</tt>, <tt>rcu_for_each_node_breadth_first</tt>,
+<tt>rcu_for_each_nonleaf_node_breadth_first()</tt>, and
+<tt>rcu_for_each_leaf_node()</tt> function and macros:
+
+<pre>
+  1 static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
+  2 {
+  3   return &amp;rsp-&gt;node[0];
+  4 }
+  5
+  6 #define rcu_for_each_node_breadth_first(rsp, rnp) \
+  7   for ((rnp) = &amp;(rsp)-&gt;node[0]; \
+  8        (rnp) &lt; &amp;(rsp)-&gt;node[NUM_RCU_NODES]; (rnp)++)
+  9
+ 10 #define rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) \
+ 11   for ((rnp) = &amp;(rsp)-&gt;node[0]; \
+ 12        (rnp) &lt; (rsp)-&gt;level[NUM_RCU_LVLS - 1]; (rnp)++)
+ 13
+ 14 #define rcu_for_each_leaf_node(rsp, rnp) \
+ 15   for ((rnp) = (rsp)-&gt;level[NUM_RCU_LVLS - 1]; \
+ 16        (rnp) &lt; &amp;(rsp)-&gt;node[NUM_RCU_NODES]; (rnp)++)
+</pre>
+
+<p>The <tt>rcu_get_root()</tt> simply returns a pointer to the
+first element of the specified <tt>rcu_state</tt> structure's
+<tt>-&gt;node[]</tt> array, which is the root <tt>rcu_node</tt>
+structure.
+
+</p><p>As noted earlier, the <tt>rcu_for_each_node_breadth_first()</tt>
+macro takes advantage of the layout of the <tt>rcu_node</tt>
+structures in the <tt>rcu_state</tt> structure's
+<tt>-&gt;node[]</tt> array, performing a breadth-first traversal by
+simply traversing the array in order.
+The <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> macro operates
+similarly, but traverses only the first part of the array, thus excluding
+the leaf <tt>rcu_node</tt> structures.
+Finally, the <tt>rcu_for_each_leaf_node()</tt> macro traverses only
+the last part of the array, thus traversing only the leaf
+<tt>rcu_node</tt> structures.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	What do <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> and
+	<tt>rcu_for_each_leaf_node()</tt> do if the <tt>rcu_node</tt> tree
+	contains only a single node?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	In the single-node case,
+	<tt>rcu_for_each_nonleaf_node_breadth_first()</tt> is a no-op
+	and <tt>rcu_for_each_leaf_node()</tt> traverses the single node.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h3><a name="Summary">
+Summary</a></h3>
+
+So each flavor of RCU is represented by an <tt>rcu_state</tt> structure,
+which contains a combining tree of <tt>rcu_node</tt> and
+<tt>rcu_data</tt> structures.
+Finally, in <tt>CONFIG_NO_HZ_IDLE</tt> kernels, each CPU's dyntick-idle
+state is tracked by an <tt>rcu_dynticks</tt> structure.
+
+If you made it this far, you are well prepared to read the code
+walkthroughs in the other articles in this series.
+
+<h3><a name="Acknowledgments">
+Acknowledgments</a></h3>
+
+I owe thanks to Cyrill Gorcunov, Mathieu Desnoyers, Dhaval Giani, Paul
+Turner, Abhishek Srivastava, Matt Kowalczyk, and Serge Hallyn
+for helping me get this document into a more human-readable state.
+
+<h3><a name="Legal Statement">
+Legal Statement</a></h3>
+
+<p>This work represents the view of the author and does not necessarily
+represent the view of IBM.
+
+</p><p>Linux is a registered trademark of Linus Torvalds.
+
+</p><p>Other company, product, and service names may be trademarks or
+service marks of others.
+
+</body></html>
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+       style="font-size:324px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;font-family:Courier">-&gt;tails[RCU_DONE_TAIL]</text>
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+    <text
+       xml:space="preserve"
+       x="225"
+       y="2925"
+       font-style="normal"
+       font-weight="bold"
+       font-size="324"
+       id="text68"
+       style="font-size:324px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;font-family:Courier">-&gt;tails[RCU_WAIT_TAIL]</text>
+    <!-- Text -->
+    <text
+       xml:space="preserve"
+       x="225"
+       y="4050"
+       font-style="normal"
+       font-weight="bold"
+       font-size="324"
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+       style="font-size:324px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;font-family:Courier">-&gt;tails[RCU_NEXT_READY_TAIL]</text>
+    <!-- Text -->
+    <text
+       xml:space="preserve"
+       x="225"
+       y="5175"
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+       font-weight="bold"
+       font-size="324"
+       id="text72"
+       style="font-size:324px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;font-family:Courier">-&gt;tails[RCU_NEXT_TAIL]</text>
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+    <text
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+       id="text74">CB 1</text>
+    <!-- Text -->
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+       x="11475"
+       y="1800"
+       fill="#000000"
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+       font-weight="normal"
+       font-size="324"
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+       id="text78">CB 3</text>
+    <!-- Text -->
+    <text
+       xml:space="preserve"
+       x="11475"
+       y="8550"
+       fill="#000000"
+       font-family="Helvetica"
+       font-style="normal"
+       font-weight="normal"
+       font-size="324"
+       text-anchor="middle"
+       id="text80">next</text>
+    <!-- Text -->
+    <text
+       xml:space="preserve"
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+       y="10800"
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+       font-family="Helvetica"
+       font-style="normal"
+       font-weight="normal"
+       font-size="324"
+       text-anchor="middle"
+       id="text82">CB 4</text>
+    <!-- Text -->
+    <text
+       xml:space="preserve"
+       x="11475"
+       y="11925"
+       fill="#000000"
+       font-family="Helvetica"
+       font-style="normal"
+       font-weight="normal"
+       font-size="324"
+       text-anchor="middle"
+       id="text84">next</text>
+    <!-- Text -->
+    <text
+       xml:space="preserve"
+       x="11475"
+       y="4050"
+       fill="#000000"
+       font-family="Helvetica"
+       font-style="normal"
+       font-weight="normal"
+       font-size="324"
+       text-anchor="middle"
+       id="text86">CB 2</text>
+    <!-- Text -->
+    <text
+       xml:space="preserve"
+       x="11475"
+       y="5175"
+       fill="#000000"
+       font-family="Helvetica"
+       font-style="normal"
+       font-weight="normal"
+       font-size="324"
+       text-anchor="middle"
+       id="text88">next</text>
+  </g>
+</svg>