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@linux.vnet.ibm.com>

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@linux.vnet.ibm.com>
c64c4b0f · Paul E. McKenney · 4b689330 · c64c4b0f · c64c4b0f
Commit c64c4b0f authored Nov 06, 2015 by Paul E. McKenney
2 changed files
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
 <!-- 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.

--- 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.
@@ -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.