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Message-ID: <fbd3bebc-db09-4224-bdf9-b44304bf55b8@gmail.com>
Date: Wed, 3 Jul 2024 12:55:57 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Yang Yang <yang.yang@...o.com>, Ming Lei <ming.lei@...hat.com>,
Jens Axboe <axboe@...nel.dk>, Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH v5] sbitmap: fix io hung due to race on
sbitmap_word::cleared
On 7/3/24 03:28, Yang Yang wrote:
> Configuration for sbq:
> depth=64, wake_batch=6, shift=6, map_nr=1
>
> 1. There are 64 requests in progress:
> map->word = 0xFFFFFFFFFFFFFFFF
> 2. After all the 64 requests complete, and no more requests come:
> map->word = 0xFFFFFFFFFFFFFFFF, map->cleared = 0xFFFFFFFFFFFFFFFF
> 3. Now two tasks try to allocate requests:
> T1: T2:
> __blk_mq_get_tag .
> __sbitmap_queue_get .
> sbitmap_get .
> sbitmap_find_bit .
> sbitmap_find_bit_in_word .
> __sbitmap_get_word -> nr=-1 __blk_mq_get_tag
> sbitmap_deferred_clear __sbitmap_queue_get
> /* map->cleared=0xFFFFFFFFFFFFFFFF */ sbitmap_find_bit
> if (!READ_ONCE(map->cleared)) sbitmap_find_bit_in_word
> return false; __sbitmap_get_word -> nr=-1
> mask = xchg(&map->cleared, 0) sbitmap_deferred_clear
> atomic_long_andnot() /* map->cleared=0 */
> if (!(map->cleared))
> return false;
> /*
> * map->cleared is cleared by T1
> * T2 fail to acquire the tag
> */
>
> 4. T2 is the sole tag waiter. When T1 puts the tag, T2 cannot be woken
> up due to the wake_batch being set at 6. If no more requests come, T1
> will wait here indefinitely.
>
> This patch achieves two purposes:
> 1. Check on ->cleared and update on both ->cleared and ->word need to
> be done atomically, and using spinlock could be the simplest solution.
> So revert commit 661d4f55a794 ("sbitmap: remove swap_lock"), which
> may cause potential race.
>
> 2. Add extra check in sbitmap_deferred_clear(), to identify whether
> ->word has free bits.
>
> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
Is it blamed right? Considering that the revert alone doesn't fix
the problem, it sounds like the 2nd step might need to be ported
to kernels even without the blamed commit.
--
Pavel Begunkov
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