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Subject[PATCH v2 tip/core/rcu 7/8] documentation: Update RCU requirements based on expedited changes
Date
Because RCU-sched expedited grace periods now use IPIs and interact
with rcu_read_unlock(), it is no longer sufficient to disable preemption
across RCU read-side critical sections that acquire and hold scheduler
locks. It is now necessary to instead disable interrupts. This commit
documents this change.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/RCU/Design/Requirements/Requirements.html | 12 ++++++++++--
Documentation/RCU/Design/Requirements/Requirements.htmlx | 10 +++++++++-
2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 96cdcf7195d5..a725f9900ec8 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -1,5 +1,5 @@
<!-- DO NOT HAND EDIT. -->
-<!-- Instead, edit Requirements.htmlx and run 'sh htmlqqz.sh Requirements' -->
+<!-- Instead, edit Documentation/RCU/Design/Requirements/Requirements.htmlx and run 'sh htmlqqz.sh Documentation/RCU/Design/Requirements/Requirements' -->
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
@@ -1957,7 +1957,7 @@ the runqueue locks and the priority-inheritance locks
in the middle of an outermost RCU read-side critical section unless either
(1)&nbsp;it releases them before exiting that same
RCU read-side critical section, or
-(2)&nbsp;preemption is disabled across
+(2)&nbsp;interrupts are disabled across
that entire RCU read-side critical section.
This same prohibition also applies (recursively!) to any lock that is acquired
while holding any lock to which this prohibition applies.
@@ -1966,6 +1966,14 @@ Adhering to this rule prevents preemptible RCU from invoking
priority-inheritance locks are held, thus avoiding deadlock.

<p>
+Prior to v4.4, it was only necessary to disable preemption across
+RCU read-side critical sections that acquired scheduler locks.
+In v4.4, expedited grace periods started using IPIs, and these
+IPIs could force a <tt>rcu_read_unlock()</tt> to take the slowpath.
+Therefore, this expedited-grace-period change required disabling of
+interrupts, not just preemption.
+
+<p>
For RCU's part, the preemptible-RCU <tt>rcu_read_unlock()</tt>
implementation must be written carefully to avoid similar deadlocks.
In particular, <tt>rcu_read_unlock()</tt> must tolerate an
diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx
index 2d0cd90987f6..3a97ba490c42 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.htmlx
+++ b/Documentation/RCU/Design/Requirements/Requirements.htmlx
@@ -2124,7 +2124,7 @@ the runqueue locks and the priority-inheritance locks
in the middle of an outermost RCU read-side critical section unless either
(1)&nbsp;it releases them before exiting that same
RCU read-side critical section, or
-(2)&nbsp;preemption is disabled across
+(2)&nbsp;interrupts are disabled across
that entire RCU read-side critical section.
This same prohibition also applies (recursively!) to any lock that is acquired
while holding any lock to which this prohibition applies.
@@ -2133,6 +2133,14 @@ Adhering to this rule prevents preemptible RCU from invoking
priority-inheritance locks are held, thus avoiding deadlock.

<p>
+Prior to v4.4, it was only necessary to disable preemption across
+RCU read-side critical sections that acquired scheduler locks.
+In v4.4, expedited grace periods started using IPIs, and these
+IPIs could force a <tt>rcu_read_unlock()</tt> to take the slowpath.
+Therefore, this expedited-grace-period change required disabling of
+interrupts, not just preemption.
+
+<p>
For RCU's part, the preemptible-RCU <tt>rcu_read_unlock()</tt>
implementation must be written carefully to avoid similar deadlocks.
In particular, <tt>rcu_read_unlock()</tt> must tolerate an
--
2.5.2


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