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rmlock: Add a required compiler membar to the rlock slow path

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The tracker flags need to be loaded only after the tracker is removed
from its per-CPU queue.  Otherwise, readers may fail to synchronize with
pending writers attempting to propagate priority to active readers, and
readers and writers deadlock on each other.  This was observed in a
stable/12-based armv7 kernel where the compiler had reordered the load
of rmp_flags to before the stores updating the queue.

Reviewed by:	rlibby, scottl
Discussed with:	kib
Sponsored by:	Rubicon Communications, LLC ("Netgate")
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D28821
This commit is contained in:
Mark Johnston 2021-02-23 21:15:50 -05:00
parent 6ab923cbca
commit 1d44514fcd
1 changed files with 5 additions and 1 deletions
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6
sys/kern/kern_rmlock.c
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@ -362,7 +362,11 @@ _rm_rlock_hard(struct rmlock *rm, struct rm_priotracker *tracker, int trylock)
/* Remove our tracker from the per-cpu list. */
rm_tracker_remove(pc, tracker);
/* Check to see if the IPI granted us the lock after all. */
/*
* Check to see if the IPI granted us the lock after all. The load of
* rmp_flags must happen after the tracker is removed from the list.
*/
__compiler_membar();
if (tracker->rmp_flags) {
/* Just add back tracker - we hold the lock. */
rm_tracker_add(pc, tracker);