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

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Date:   Mon, 11 Jul 2022 16:26:23 +0200
From:   Jan Kara <jack@...e.cz>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     axboe@...nel.dk, asml.silence@...il.com, osandov@...com,
        jack@...e.cz, kbusch@...nel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yukuai3@...wei.com,
        yi.zhang@...wei.com
Subject: Re: [PATCH RFC v3 2/3] sbitmap: fix invalid wakeup on the wrong
 waitqueue

On Sun 10-07-22 12:21:59, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.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@...wei.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).

								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
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR