Re: [PATCH RFC v3 2/3] sbitmap: fix invalid wakeup on the wrong waitqueue

[Yu Kuai <yukuai3@huawei.com>]

Hi!

在 2022/07/11 22:26, Jan Kara 写道:
> On Sun 10-07-22 12:21:59, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@huawei.com>
>>
>> For example, 2 * wake_batch tags are put, while only wake_batch threads
>> are woken:
>>
>> __sbq_wake_up
>>   atomic_cmpxchg -> reset wait_cnt
>> 			__sbq_wake_up -> decrease wait_cnt
>> 			...
>> 			__sbq_wake_up -> wait_cnt is decreased to 0 again
>> 			 atomic_cmpxchg
>> 			 sbq_index_atomic_inc -> increase wake_index
>> 			 wake_up_nr -> wake up and waitqueue might be empty
>>   sbq_index_atomic_inc -> increase again, one waitqueue is skipped
>>   wake_up_nr -> invalid wake up because old wakequeue might be empty
>>
>> To fix the problem, increasing 'wake_index' before resetting 'wait_cnt'.
>>
>> Fixes: 88459642cba4 ("blk-mq: abstract tag allocation out into sbitmap library")
>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> 
> This patch and the following one look sane to me but please merge them to a
> single patch. They fix the same race of two concurrent wakers just with a
> slightly different timing so there isn't a point in having two patches for
> this (in particular changes in this patch are difficult to reason about
> when we know the result is still buggy).

Ok, I'll merge them.

Thanks,
Kuai
> 
> 								Honza
> 
>> ---
>>   lib/sbitmap.c | 45 +++++++++++++++++++++++----------------------
>>   1 file changed, 23 insertions(+), 22 deletions(-)
>>
>> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
>> index b46fce1beb3a..57095dd88a33 100644
>> --- a/lib/sbitmap.c
>> +++ b/lib/sbitmap.c
>> @@ -616,32 +616,33 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
>>   		return false;
>>   
>>   	wait_cnt = atomic_dec_return(&ws->wait_cnt);
>> -	if (wait_cnt <= 0) {
>> -		int ret;
>> +	if (wait_cnt > 0)
>> +		return false;
>>   
>> -		wake_batch = READ_ONCE(sbq->wake_batch);
>> +	/*
>> +	 * For concurrent callers of this, callers should call this function
>> +	 * again to wakeup a new batch on a different 'ws'.
>> +	 */
>> +	if (wait_cnt < 0)
>> +		return true;
>>   
>> -		/*
>> -		 * Pairs with the memory barrier in sbitmap_queue_resize() to
>> -		 * ensure that we see the batch size update before the wait
>> -		 * count is reset.
>> -		 */
>> -		smp_mb__before_atomic();
>> +	wake_batch = READ_ONCE(sbq->wake_batch);
>>   
>> -		/*
>> -		 * For concurrent callers of this, the one that failed the
>> -		 * atomic_cmpxhcg() race should call this function again
>> -		 * to wakeup a new batch on a different 'ws'.
>> -		 */
>> -		ret = atomic_cmpxchg(&ws->wait_cnt, wait_cnt, wake_batch);
>> -		if (ret == wait_cnt) {
>> -			sbq_index_atomic_inc(&sbq->wake_index);
>> -			wake_up_nr(&ws->wait, wake_batch);
>> -			return false;
>> -		}
>> +	/*
>> +	 * Pairs with the memory barrier in sbitmap_queue_resize() to
>> +	 * ensure that we see the batch size update before the wait
>> +	 * count is reset.
>> +	 */
>> +	smp_mb__before_atomic();
>>   
>> -		return true;
>> -	}
>> +	/*
>> +	 * Increase wake_index before updating wait_cnt, otherwise concurrent
>> +	 * callers can see valid wait_cnt in old waitqueue, which can cause
>> +	 * invalid wakeup on the old waitqueue.
>> +	 */
>> +	sbq_index_atomic_inc(&sbq->wake_index);
>> +	atomic_set(&ws->wait_cnt, wake_batch);
>> +	wake_up_nr(&ws->wait, wake_batch);
>>   
>>   	return false;
>>   }
>> -- 
>> 2.31.1
>>