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documentation: Update RCU requirements based on expedited changes

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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>
This commit is contained in:
Paul E. McKenney 2015-11-06 23:05:32 -08:00
parent 4b689330b1
commit c64c4b0f9a
2 changed files with 19 additions and 3 deletions
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12
Documentation/RCU/Design/Requirements/Requirements.html
View File

@ -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.
@ -1965,6 +1965,14 @@ Adhering to this rule prevents preemptible RCU from invoking
<tt>rcu_read_unlock_special()</tt> while either runqueue or
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.

10
Documentation/RCU/Design/Requirements/Requirements.htmlx
View File

@ -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.
@ -2132,6 +2132,14 @@ Adhering to this rule prevents preemptible RCU from invoking
<tt>rcu_read_unlock_special()</tt> while either runqueue or
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.