linux-kernel - [PATCH tip/core/rcu 7/8] documentation: Update RCU requirements based on expedited changes

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Message-Id: <1449273026-23633-7-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Fri,  4 Dec 2015 15:50:25 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 7/8] documentation: Update RCU requirements based on expedited changes

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@...ux.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 f5bf27228192..9aa8f2c402f5 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 4082e588baa7..de86311e638e 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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