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Date:   Wed, 22 Jan 2020 16:59:39 +0000
From:   Will Deacon <will@kernel.org>
To:     Qian Cai <cai@lca.pw>
Cc:     mingo@redhat.com, peterz@infradead.org, elver@google.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] locking/osq_lock: fix a data race in osq_wait_next
Message-ID: <20200122165938.GA16974@willie-the-truck>
References: <20200122163857.4605-1-cai@lca.pw>
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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
> 
> 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().
> 
> 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;
>  		}

I don't understand this; 'next' is a local variable.

Not keen on the onslaught of random "add a READ_ONCE() to shut the
sanitiser up" patches we're going to get from kcsan :(

Will