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From: Mark Johnston <markj@FreeBSD.org>
Subject: git: 20f9e2a723f5 - releng/13.0 - rmlock: Add a required compiler
membar to the rlock slow path
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The branch releng/13.0 has been updated by markj:
URL: https://cgit.FreeBSD.org/src/commit/?id=20f9e2a723f5f560d6219e28f36dd3b8f8561b3a
commit 20f9e2a723f5f560d6219e28f36dd3b8f8561b3a
Author: Mark Johnston <markj@FreeBSD.org>
AuthorDate: 2021-02-24 02:15:50 +0000
Commit: Mark Johnston <markj@FreeBSD.org>
CommitDate: 2021-03-03 01:16:54 +0000
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.
Approved by: re (gjb)
Reviewed by: rlibby, scottl
Discussed with: kib
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D28821
(cherry picked from commit 1d44514fcd68809cfd493a7352ace29ddad443d6)
(cherry picked from commit c29d3ecdc8b3903d812c0467319ced6f85872d0e)
---
sys/kern/kern_rmlock.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
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);