Adding upstream version 6.1.76.upstream/6.1.76upstream

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

1 files changed, 91 insertions, 0 deletions

diff --git a/Documentation/RCU/rcu.rst b/Documentation/RCU/rcu.rst
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+.. _rcu_doc:
+
+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 listRCU.rst for more
+information on using RCU with linked lists.
+
+Frequently Asked Questions
+--------------------------
+
+- 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.
+
+- 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.
+
+- 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 UP.rst for more information.
+
+- 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", "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).
+
+- What guidelines should I follow when writing code that uses RCU?
+
+  See checklist.rst.
+
+- Why the name "RCU"?
+
+  "RCU" stands for "read-copy update".
+  listRCU.rst has more information on where this name came from, search
+  for "read-copy update" to find it.
+
+- 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 Documentation/RCU/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 (https://liburcu.org/).
+
+- 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.
+
+- Where can I find more information on RCU?
+
+  See the Documentation/RCU/RTFP.txt file.
+  Or point your browser at (http://www.rdrop.com/users/paulmck/RCU/).