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

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