linux-kernel - Re: [PATCH v6] sbitmap: fix io hung due to race on sbitmap

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Message-ID: <ead047aa-d9dc-4b2f-869f-610b309b5092@vivo.com>
Date: Thu, 11 Jul 2024 20:48:03 +0800
From: YangYang <yang.yang@...o.com>
To: Bart Van Assche <bvanassche@....org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Jens Axboe <axboe@...nel.dk>,
 Ming Lei <ming.lei@...hat.com>, Omar Sandoval <osandov@...com>,
 linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH v6] sbitmap: fix io hung due to race on
 sbitmap_word::cleared

On 2024/7/11 3:54, Bart Van Assche wrote:
> On 7/9/24 11:56 PM, Yang Yang wrote:
>> +    /**
>> +     * @swap_lock: Held while swapping word <-> cleared
>> +     */
>> +    spinlock_t swap_lock;
> 
> Why is only swapping 'word' with 'cleared' protected by the spinlock?
> If all 'cleared' changes would be protected by this spinlock then
> that would allow to eliminate the expensive xchg() call from
> sbitmap_deferred_clear().

The spinlock was initially introduced in ea86ea2cdced ("sbitmap:
ammortize cost of clearing bits").
I think if all 'cleared' changes are protected by the spinlock, the
overhead of clearing tags would definitely increase.

Thanks.

> 
> Thanks,
> 
> Bart.