diff options
authorMark Johnston <markj@FreeBSD.org>2021-02-24 02:15:50 +0000
committerMark Johnston <markj@FreeBSD.org>2021-02-24 02:17:12 +0000
commit1d44514fcd68809cfd493a7352ace29ddad443d6 (patch)
parent6ab923cbca8759503a08683a5978b9ebf5efd607 (diff)
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
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);