linux-kernel - Re: [PATCH v7] sbitmap: fix io hung due to race on sbitmap

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Message-ID: <37053ba7-68bb-4afc-88cd-8ceac30e2f85@vivo.com>
Date: Tue, 16 Jul 2024 11:48:01 +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 v7] sbitmap: fix io hung due to race on
 sbitmap_word::cleared

On 2024/7/16 1:26, Bart Van Assche wrote:
> On 7/15/24 2:11 AM, Yang Yang wrote:
>> +
>> +    /**
>> +     * @swap_lock: serializes simultaneous updates of ->word and ->cleared
>> +     */
>> +    spinlock_t swap_lock;
>>   } ____cacheline_aligned_in_smp;
> 
> Thank you for having updated this comment.
> 
>> -static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
>> +static inline bool sbitmap_deferred_clear(struct sbitmap_word *map,
>> +        unsigned int depth, unsigned int alloc_hint, bool wrap)
>>   {
>> -    unsigned long mask;
>> +    unsigned long mask, word_mask;
>> +    bool ret = false;
>> -    if (!READ_ONCE(map->cleared))
>> -        return false;
>> +    guard(spinlock_irqsave)(&map->swap_lock);
>> +
>> +    if (!map->cleared) {
>> +        if (depth > 0) {
>> +            word_mask = (~0UL) >> (BITS_PER_LONG - depth);
>> +            /*
>> +             * The current behavior is to always retry after moving
>> +             * ->cleared to word, and we change it to retry in case
>> +             * of any free bits. To avoid an infinite loop, we need
>> +             * to take wrap & alloc_hint into account, otherwise a
>> +             * soft lockup may occur.
>> +             */
>> +            if (!wrap && alloc_hint)
>> +                word_mask &= ~((1UL << alloc_hint) - 1);
>> +
>> +            if ((READ_ONCE(map->word) & word_mask) == word_mask)
>> +                ret = false;
>> +            else
>> +                ret = true;
>> +        }
>> +
>> +        return ret;
>> +    }
> 
> Now that guard()() is being used, the local variable 'ret' can be eliminated. The if 
> (READ_ONCE() ...) statement can be changed into the
> following: return (READ_ONCE(map->word) & word_mask) != word_mask;
> and "return ret;" can be changed into "return false;". Additionally,
> the indentation depth can be reduced by changing "if (depth > 0) {" ...
> into "if (depth == 0) return false;".

This looks much better, I will make these changes in the next version.

Thanks.

> 
> Otherwise this patch looks good to me.
> 
> Thanks,
> 
> Bart.