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author | Mark Johnston <markj@FreeBSD.org> | 2021-02-24 02:15:50 +0000 |
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committer | Mark Johnston <markj@FreeBSD.org> | 2021-02-24 02:17:12 +0000 |
commit | 1d44514fcd68809cfd493a7352ace29ddad443d6 (patch) | |
tree | a01c4ace8173624b4eb85a9590e535cec793b984 | |
parent | 6ab923cbca8759503a08683a5978b9ebf5efd607 (diff) | |
download | src-1d44514fcd68809cfd493a7352ace29ddad443d6.tar.gz src-1d44514fcd68809cfd493a7352ace29ddad443d6.zip |
rmlock: Add a required compiler membar to the rlock slow path
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
-rw-r--r-- | sys/kern/kern_rmlock.c | 6 |
1 files changed, 5 insertions, 1 deletions
diff --git a/sys/kern/kern_rmlock.c b/sys/kern/kern_rmlock.c index 401445d3d03f..9135709d88cf 100644 --- a/sys/kern/kern_rmlock.c +++ b/sys/kern/kern_rmlock.c @@ -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); |