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Message-ID: <fceb1465-d1b3-42bc-ace9-60a93ff4666f@vivo.com>
Date: Tue, 2 Jul 2024 15:57:00 +0800
From: YangYang <yang.yang@...o.com>
To: Ming Lei <ming.lei@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Jens Axboe <axboe@...nel.dk>,
 Pavel Begunkov <asml.silence@...il.com>, linux-kernel@...r.kernel.org,
 linux-block@...r.kernel.org
Subject: Re: [PATCH v4] sbitmap: fix io hung due to race on
 sbitmap_word::cleared

On 2024/7/2 11:34, Ming Lei wrote:
> On Mon, Jun 17, 2024 at 10:45:51AM +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.
>>
>> 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")
>> Signed-off-by: Yang Yang <yang.yang@...o.com>
>>
>> ---
>> Changes from v3:
>>    - Add more arguments to sbitmap_deferred_clear(), for those who
>>      don't care about the return value, just pass 0
>>    - Consider the situation when using sbitmap_get_shallow()
>>    - Consider the situation when ->round_robin is true
>>    - Modify commit message
>> Changes from v2:
>>    - Modify commit message by suggestion
>>    - Add extra check in sbitmap_deferred_clear() by suggestion
>> Changes from v1:
>>    - simply revert commit 661d4f55a794 ("sbitmap: remove swap_lock")
>> ---
>>   include/linux/sbitmap.h |  5 +++++
>>   lib/sbitmap.c           | 45 ++++++++++++++++++++++++++++++++---------
>>   2 files changed, 41 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
>> index d662cf136021..ec0b0e73c906 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_lock: Held while swapping word <-> cleared
>> +	 */
>> +	spinlock_t swap_lock;
>>   } ____cacheline_aligned_in_smp;
>>   
>>   /**
>> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
>> index 1e453f825c05..9bd85a9b74b9 100644
>> --- a/lib/sbitmap.c
>> +++ b/lib/sbitmap.c
>> @@ -60,12 +60,32 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
>>   /*
>>    * See if we have deferred clears that we can batch move
>>    */
>> -static inline bool sbitmap_deferred_clear(struct sbitmap_word *map)
>> -{
>> -	unsigned long mask;
>> +static inline bool sbitmap_deferred_clear(struct sbitmap_word *map,
>> +		unsigned int depth, unsigned int alloc_hint, bool wrap)
>> +{
>> +	unsigned long mask, flags, word_mask;
>> +	bool ret = false;
>> +
>> +	spin_lock_irqsave(&map->swap_lock, flags);
>> +
>> +	if (!map->cleared) {
>> +		if (depth > 0) {
>> +#if BITS_PER_LONG == 64
>> +			word_mask = U64_MAX >> (BITS_PER_LONG - depth);
>> +#else
>> +			word_mask = U32_MAX >> (BITS_PER_LONG - depth);
>> +#endif
> 
> Can we avoid the above conditional compiling by the following way?
> 
> 	word_mask = (~0UL) >> (BITS_PER_LONG - depth);

Sure, this looks better.

> 
>> +			if (!wrap && alloc_hint)
>> +				word_mask &= ~((1UL << alloc_hint) - 1);
> 
> The current behavior is to always retry after moving ->cleared to
> word, and we change it to retry in case of any free bits. So here
> is it for avoiding dead loop by taking wrap & alloc_hint into
> account? If yes, this way looks fine, but I'd suggest to document
> this change.

Yes. Thank you for the suggestion. I will include the following
descriptions in the next version.
/*
  * 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 dead loop, we need to take
  * wrap & alloc_hint into account. Without this, a soft
  * lockup was detected in our test environment.
  */

Thanks.

> 
> Thanks,
> Ming
> 
> 


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