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Commit c64c4b0f authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 4b689330

Documentation/RCU/Design/Requirements/Requirements.html

+10 −2
Original line number Diff line number Diff line 
<!-- 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.

Documentation/RCU/Design/Requirements/Requirements.htmlx

+9 −1
Original line number Diff line number Diff line
@@ -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.