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From: Peter Zijlstra <peterz@infradead.org>
To: Qian Cai <cai@lca.pw>
Cc: mingo@redhat.com, will@kernel.org, elver@google.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] locking/osq_lock: fix a data race in osq_wait_next
Date: Wed, 22 Jan 2020 18:09:44 +0100
Message-ID: <20200122170944.GZ14879@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200122163857.4605-1-cai@lca.pw>

On Wed, Jan 22, 2020 at 11:38:57AM -0500, Qian Cai wrote:
> KCSAN complains,
> 
>  write (marked) to 0xffff941ca3b3be00 of 8 bytes by task 670 on cpu 6:
>   osq_lock+0x24c/0x340
>   __mutex_lock+0x277/0xd20
>   mutex_lock_nested+0x31/0x40
>   memcg_create_kmem_cache+0x2e/0x190
>   memcg_kmem_cache_create_func+0x40/0x80
>   process_one_work+0x54c/0xbe0
>   worker_thread+0x80/0x650
>   kthread+0x1e0/0x200
>   ret_from_fork+0x27/0x50
> 
>  read to 0xffff941ca3b3be00 of 8 bytes by task 703 on cpu 44:
>   osq_lock+0x18e/0x340
>   __mutex_lock+0x277/0xd20
>   mutex_lock_nested+0x31/0x40
>   memcg_create_kmem_cache+0x2e/0x190
>   memcg_kmem_cache_create_func+0x40/0x80
>   process_one_work+0x54c/0xbe0
>   worker_thread+0x80/0x650
>   kthread+0x1e0/0x200
>   ret_from_fork+0x27/0x50

That's useless gibberish, at the very least run it through some decode
so we get line numbers.

> which points to those lines in osq_wait_next(),
> 
>   next = xchg(&node->next, NULL);
>   if (next)
> 	break;
> 
> Since only the read is outside of critical sections, fixed it by adding
> a READ_ONCE().

What?!?! Did you actually read what you wrote?

Also, you have to stop calling things fixes unless you can prove (and
explain) there is an actual problem.

> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
>  kernel/locking/osq_lock.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 6ef600aa0f47..8f565165019a 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -77,7 +77,7 @@ osq_wait_next(struct optimistic_spin_queue *lock,
>  		 */
>  		if (node->next) {
>  			next = xchg(&node->next, NULL);
> -			if (next)
> +			if (READ_ONCE(next))
>  				break;
>  		}
>  

This seems to suggest you ought to maybe brush up on your C skills.

      parent reply index

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-22 16:38 Qian Cai
2020-01-22 16:59 ` Will Deacon
2020-01-22 17:08   ` Qian Cai
2020-01-22 22:38     ` Marco Elver
2020-01-22 23:54       ` Qian Cai
2020-01-23  9:39         ` Peter Zijlstra
2020-01-28  3:11           ` Qian Cai
2020-01-28 11:46             ` Marco Elver
2020-01-28 12:53               ` Qian Cai
2020-01-28 16:52               ` Peter Zijlstra
2020-01-28 16:56               ` Peter Zijlstra
2020-01-29  0:22                 ` Paul E. McKenney
2020-01-29 15:29                   ` Marco Elver
2020-01-29 18:40                     ` Peter Zijlstra
2020-01-30 13:39                       ` Marco Elver
2020-01-30 13:48                         ` Peter Zijlstra
2020-01-31  3:32                           ` Qian Cai
2020-01-29 18:49                   ` Peter Zijlstra
2020-01-29 19:26                     ` Paul E. McKenney
2020-01-23  9:36       ` Peter Zijlstra
2020-01-28  3:12         ` Qian Cai
2020-01-28  8:18           ` Marco Elver
2020-01-28 10:10             ` Qian Cai
2020-01-28 10:29               ` Marco Elver
2020-01-22 17:09 ` Peter Zijlstra [this message]

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