From: Qian Cai <cai@lca.pw>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Will Deacon <will@kernel.org>, Elver Marco <elver@google.com>,
LKML <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>
Subject: Re: [PATCH -next v2] locking/osq_lock: annotate a data race in osq_lock
Date: Sat, 9 May 2020 09:01:53 -0400 [thread overview]
Message-ID: <9FBC2A17-2821-49F7-9A0F-5E41116BBC70@lca.pw> (raw)
In-Reply-To: <20200509043308.GL2869@paulmck-ThinkPad-P72>
> 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
next prev parent reply other threads:[~2020-05-09 13:01 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-11 13:54 [PATCH -next v2] locking/osq_lock: annotate a data race in osq_lock Qian Cai
2020-05-08 20:59 ` Qian Cai
2020-05-09 4:33 ` Paul E. McKenney
2020-05-09 13:01 ` Qian Cai [this message]
2020-05-09 16:12 ` Paul E. McKenney
2020-05-09 16:53 ` Qian Cai
2020-05-09 21:36 ` Paul E. McKenney
2020-05-11 15:58 ` Will Deacon
2020-05-11 16:43 ` Paul E. McKenney
2020-05-11 16:52 ` Will Deacon
2020-05-11 17:29 ` Paul E. McKenney
2020-05-11 17:34 ` Will Deacon
2020-05-11 18:07 ` Paul E. McKenney
2020-05-11 16:44 ` Qian Cai
2020-05-11 16:54 ` Will Deacon
2020-05-11 17:10 ` Qian Cai
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