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: Fri, 8 May 2020 16:59:05 -0400 [thread overview]
Message-ID: <2C8BF141-5384-475F-B713-6D473557C65C@lca.pw> (raw)
In-Reply-To: <1581429255-12542-1-git-send-email-cai@lca.pw>
> 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/
> ---
>
> 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:[~2020-05-08 20:59 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 [this message]
2020-05-09 4:33 ` Paul E. McKenney
2020-05-09 13:01 ` Qian Cai
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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