Date: Wed, 24 Feb 2021 02:18:09 GMT From: Mark Johnston <markj@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: git: 1d44514fcd68 - main - rmlock: Add a required compiler membar to the rlock slow path Message-ID: <202102240218.11O2I9tY096585@gitrepo.freebsd.org>
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The branch main has been updated by markj: URL: https://cgit.FreeBSD.org/src/commit/?id=1d44514fcd68809cfd493a7352ace29ddad443d6 commit 1d44514fcd68809cfd493a7352ace29ddad443d6 Author: Mark Johnston <markj@FreeBSD.org> AuthorDate: 2021-02-24 02:15:50 +0000 Commit: Mark Johnston <markj@FreeBSD.org> CommitDate: 2021-02-24 02:17:12 +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. Reviewed by: rlibby, scottl Discussed with: kib Sponsored by: Rubicon Communications, LLC ("Netgate") MFC after: 1 week Differential Revision: https://reviews.freebsd.org/D28821 --- 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);
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