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

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Date: Tue, 4 Jun 2024 10:19:01 +0800
From: YangYang <yang.yang@...o.com>
To: Ming Lei <ming.lei@...hat.com>
Cc: Jens Axboe <axboe@...nel.dk>, Andrew Morton <akpm@...ux-foundation.org>,
 Pavel Begunkov <asml.silence@...il.com>, linux-kernel@...r.kernel.org,
 linux-block@...r.kernel.org
Subject: Re: [RFC PATCH] sbitmap: fix io hung due to race on
 sbitmap_word::cleared

On 2024/6/4 9:23, Ming Lei wrote:
> On Mon, May 27, 2024 at 12:26:50PM +0800, 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.
>>
>> Fix this issue by adding a new flag swap_inprogress to indicate whether
>> the swap is ongoing.
>>
>> Fixes: 661d4f55a794 ("sbitmap: remove swap_lock")
> 
> Indeed, commit 661d4f55a794 ("sbitmap: remove swap_lock") causes this
> issue, and ->cleared and ->word should have been checked & updated
> atomically.
> 
>> Signed-off-by: Yang Yang <yang.yang@...o.com>
>> ---
>>   include/linux/sbitmap.h |  5 +++++
>>   lib/sbitmap.c           | 22 ++++++++++++++++++++--
>>   2 files changed, 25 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
>> index d662cf136021..b88a9e4997ab 100644
>> --- a/include/linux/sbitmap.h
>> +++ b/include/linux/sbitmap.h
>> @@ -36,6 +36,11 @@ struct sbitmap_word {
>>   	 * @cleared: word holding cleared bits
>>   	 */
>>   	unsigned long cleared ____cacheline_aligned_in_smp;
>> +
>> +	/**
>> +	 * @swap_inprogress: set to 1 when swapping word <-> cleared
>> +	 */
>> +	atomic_t swap_inprogress;
>>   } ____cacheline_aligned_in_smp;
>>   
>>   /**
>> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
>> index 1e453f825c05..d4bb258fe8b0 100644
>> --- a/lib/sbitmap.c
>> +++ b/lib/sbitmap.c
>> @@ -62,10 +62,19 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
>>    */
>>   static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
>>   {
>> -	unsigned long mask;
>> +	unsigned long mask, flags;
>> +	int zero = 0;
>>   
>> -	if (!READ_ONCE(map->cleared))
>> +	if (!READ_ONCE(map->cleared)) {
>> +		if (atomic_read(&map->swap_inprogress))
>> +			goto out_wait;
>>   		return false;
>> +	}
>> +
>> +	if (!atomic_try_cmpxchg(&map->swap_inprogress, &zero, 1))
>> +		goto out_wait;
>> +
>> +	local_irq_save(flags);
>>   
>>   	/*
>>   	 * First get a stable cleared mask, setting the old mask to 0.
>> @@ -77,6 +86,15 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
>>   	 */
>>   	atomic_long_andnot(mask, (atomic_long_t *)&map->word);
>>   	BUILD_BUG_ON(sizeof(atomic_long_t) != sizeof(map->word));
>> +
>> +	atomic_set(&map->swap_inprogress, 0);
>> +	smp_mb__after_atomic();
>> +	local_irq_restore(flags);
>> +	return true;
>> +
>> +out_wait:
>> +	while (atomic_read(&map->swap_inprogress))
>> +		;
>>   	return true;
>>   }
> 
> IMO, the above fix looks a bit complicated, and I'd suggest to revert
> 661d4f55a794 ("sbitmap: remove swap_lock") if no clean & simple solution
> can be figured out.

Thanks. I will send a patch to revert that change.

> 
> Thanks,
> Ming
>