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Subject: Re: [PATCH] block: insert a general SMP memory barrier before
 wake_up_bit()
To:     Christoph Hellwig <hch@infradead.org>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>
References: <20200813024438.13170-1-jian.w.wen@oracle.com>
 <20200813073115.GA15436@infradead.org>
From:   Jacob Wen <jian.w.wen@oracle.com>
Message-ID: <0a1d9db5-dc5f-5718-048d-861385ce2832@oracle.com>
Date:   Thu, 13 Aug 2020 15:46:15 +0800
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On 8/13/20 3:31 PM, Christoph Hellwig wrote:
> On Thu, Aug 13, 2020 at 10:44:38AM +0800, Jacob Wen wrote:
>> wake_up_bit() uses waitqueue_active() that needs the explicit smp_mb().
> Sounds like the barrier should go into wake_up_bit then..

wake_up_bit() doesn't know which one to chose: smp_mb__after_atomic() or 
smp_mb().

>
>> Signed-off-by: Jacob Wen <jian.w.wen@oracle.com>
>> ---
>>   fs/block_dev.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/fs/block_dev.c b/fs/block_dev.c
>> index 0ae656e022fd..e74980848a2a 100644
>> --- a/fs/block_dev.c
>> +++ b/fs/block_dev.c
>> @@ -1175,6 +1175,7 @@ static void bd_clear_claiming(struct block_device *whole, void *holder)
>>   	/* tell others that we're done */
>>   	BUG_ON(whole->bd_claiming != holder);
>>   	whole->bd_claiming = NULL;
>> +	smp_mb();
>>   	wake_up_bit(&whole->bd_claiming, 0);
>>   }
>>   
>> -- 
>> 2.17.1
>>
> ---end quoted text---