Adding upstream version 4.19.249.upstream/4.19.249upstream

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

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+RCU Concepts
+
+
+The basic idea behind RCU (read-copy update) is to split destructive
+operations into two parts, one that prevents anyone from seeing the data
+item being destroyed, and one that actually carries out the destruction.
+A "grace period" must elapse between the two parts, and this grace period
+must be long enough that any readers accessing the item being deleted have
+since dropped their references.  For example, an RCU-protected deletion
+from a linked list would first remove the item from the list, wait for
+a grace period to elapse, then free the element.  See the listRCU.txt
+file for more information on using RCU with linked lists.
+
+
+Frequently Asked Questions
+
+o	Why would anyone want to use RCU?
+
+	The advantage of RCU's two-part approach is that RCU readers need
+	not acquire any locks, perform any atomic instructions, write to
+	shared memory, or (on CPUs other than Alpha) execute any memory
+	barriers.  The fact that these operations are quite expensive
+	on modern CPUs is what gives RCU its performance advantages
+	in read-mostly situations.  The fact that RCU readers need not
+	acquire locks can also greatly simplify deadlock-avoidance code.
+
+o	How can the updater tell when a grace period has completed
+	if the RCU readers give no indication when they are done?
+
+	Just as with spinlocks, RCU readers are not permitted to
+	block, switch to user-mode execution, or enter the idle loop.
+	Therefore, as soon as a CPU is seen passing through any of these
+	three states, we know that that CPU has exited any previous RCU
+	read-side critical sections.  So, if we remove an item from a
+	linked list, and then wait until all CPUs have switched context,
+	executed in user mode, or executed in the idle loop, we can
+	safely free up that item.
+
+	Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the
+	same effect, but require that the readers manipulate CPU-local
+	counters.  These counters allow limited types of blocking within
+	RCU read-side critical sections.  SRCU also uses CPU-local
+	counters, and permits general blocking within RCU read-side
+	critical sections.  These variants of RCU detect grace periods
+	by sampling these counters.
+
+o	If I am running on a uniprocessor kernel, which can only do one
+	thing at a time, why should I wait for a grace period?
+
+	See the UP.txt file in this directory.
+
+o	How can I see where RCU is currently used in the Linux kernel?
+
+	Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
+	"rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
+	"srcu_read_lock", "srcu_read_unlock", "synchronize_rcu",
+	"synchronize_net", "synchronize_srcu", and the other RCU
+	primitives.  Or grab one of the cscope databases from:
+
+	http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html
+
+o	What guidelines should I follow when writing code that uses RCU?
+
+	See the checklist.txt file in this directory.
+
+o	Why the name "RCU"?
+
+	"RCU" stands for "read-copy update".  The file listRCU.txt has
+	more information on where this name came from, search for
+	"read-copy update" to find it.
+
+o	I hear that RCU is patented?  What is with that?
+
+	Yes, it is.  There are several known patents related to RCU,
+	search for the string "Patent" in RTFP.txt to find them.
+	Of these, one was allowed to lapse by the assignee, and the
+	others have been contributed to the Linux kernel under GPL.
+	There are now also LGPL implementations of user-level RCU
+	available (http://liburcu.org/).
+
+o	I hear that RCU needs work in order to support realtime kernels?
+
+	Realtime-friendly RCU can be enabled via the CONFIG_PREEMPT_RCU
+	kernel configuration parameter.
+
+o	Where can I find more information on RCU?
+
+	See the RTFP.txt file in this directory.
+	Or point your browser at http://www.rdrop.com/users/paulmck/RCU/.
+
+o	What are all these files in this directory?
+
+	See 00-INDEX for the list.