Messages in this thread | | | Subject | Re: [PATCH -next v2] locking/osq_lock: annotate a data race in osq_lock | From | Qian Cai <> | Date | Sat, 9 May 2020 09:01:53 -0400 |
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> On May 9, 2020, at 12:33 AM, Paul E. McKenney <paulmck@kernel.org> wrote: > > On Fri, May 08, 2020 at 04:59:05PM -0400, Qian Cai wrote: >> >> >>> On Feb 11, 2020, at 8:54 AM, Qian Cai <cai@lca.pw> wrote: >>> >>> prev->next could be accessed concurrently as noticed by KCSAN, >>> >>> write (marked) to 0xffff9d3370dbbe40 of 8 bytes by task 3294 on cpu 107: >>> osq_lock+0x25f/0x350 >>> osq_wait_next at kernel/locking/osq_lock.c:79 >>> (inlined by) osq_lock at kernel/locking/osq_lock.c:185 >>> rwsem_optimistic_spin >>> <snip> >>> >>> read to 0xffff9d3370dbbe40 of 8 bytes by task 3398 on cpu 100: >>> osq_lock+0x196/0x350 >>> osq_lock at kernel/locking/osq_lock.c:157 >>> rwsem_optimistic_spin >>> <snip> >>> >>> Since the write only stores NULL to prev->next and the read tests if >>> prev->next equals to this_cpu_ptr(&osq_node). Even if the value is >>> shattered, the code is still working correctly. Thus, mark it as an >>> intentional data race using the data_race() macro. >>> >>> Signed-off-by: Qian Cai <cai@lca.pw> >> >> Hmm, this patch has been dropped from linux-next from some reasons. >> >> Paul, can you pick this up along with KCSAN fixes? >> >> https://lore.kernel.org/lkml/1581429255-12542-1-git-send-email-cai@lca.pw/ > > I have queued it on -rcu, but it is too late for v5.8 via the -rcu tree, > so this is v5.9 at the earliest. Plus I would need an ack from one of > the locking folks.
Peter, Will, can you give an ACK? This v2 should incorporate all the feedback from Peter,
https://lore.kernel.org/lkml/20200211124753.GP14914@hirez.programming.kicks-ass.net/
V5.9 is fine. All I care about is it is always in linux-next (so the testing bots won’t trigger this over and over again) and to be in mainline at some point in the future.
> > Thanx, Paul > >>> --- >>> >>> v2: insert some code comments. >>> >>> kernel/locking/osq_lock.c | 6 +++++- >>> 1 file changed, 5 insertions(+), 1 deletion(-) >>> >>> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c >>> index 1f7734949ac8..f733bcd99e8a 100644 >>> --- a/kernel/locking/osq_lock.c >>> +++ b/kernel/locking/osq_lock.c >>> @@ -154,7 +154,11 @@ bool osq_lock(struct optimistic_spin_queue *lock) >>> */ >>> >>> for (;;) { >>> - if (prev->next == node && >>> + /* >>> + * cpu_relax() below implies a compiler barrier which would >>> + * prevent this comparison being optimized away. >>> + */ >>> + if (data_race(prev->next == node) && >>> cmpxchg(&prev->next, node, NULL) == node) >>> break; >>> >>> -- >>> 1.8.3.1
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