Re: [PATCH RFC -next] sbitmap: fix possible io hung due to lost wakeups

[Jan Kara <jack@suse.cz>]

On Mon 20-06-22 14:24:13, Jan Kara wrote:
> On Fri 17-06-22 22:11:25, Yu Kuai wrote:
> > Currently, same waitqueue might be woken up continuously:
> > 
> > __sbq_wake_up		__sbq_wake_up
> >  sbq_wake_ptr -> assume	0
> > 			 sbq_wake_ptr -> 0
> >  atomic_dec_return
> > 			atomic_dec_return
> >  atomic_cmpxchg -> succeed
> > 			 atomic_cmpxchg -> failed
> > 			  return true
> > 
> > 			__sbq_wake_up
> > 			 sbq_wake_ptr
> > 			  atomic_read(&sbq->wake_index) -> still 0
> >  sbq_index_atomic_inc -> inc to 1
> > 			  if (waitqueue_active(&ws->wait))
> > 			   if (wake_index != atomic_read(&sbq->wake_index))
> > 			    atomic_set -> reset from 1 to 0
> >  wake_up_nr -> wake up first waitqueue
> > 			    // continue to wake up in first waitqueue
> > 
> > What's worse, io hung is possible in theory because wakeups might be
> > missed. For example, 2 * wake_batch tags are put, while only wake_batch
> > threads are worken:
> > 
> > __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, refactor to make sure waitqueues will be woken up
> > one by one,
> > 
> > Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> 
> So as far as I can tell your patch does not completely fix this race. See
> below:
> 
> > diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> > index ae4fd4de9ebe..dc2959cb188c 100644
> > --- a/lib/sbitmap.c
> > +++ b/lib/sbitmap.c
> > @@ -574,66 +574,69 @@ void sbitmap_queue_min_shallow_depth(struct sbitmap_queue *sbq,
> >  }
> >  EXPORT_SYMBOL_GPL(sbitmap_queue_min_shallow_depth);
> >  
> > -static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
> > +static void sbq_update_wake_index(struct sbitmap_queue *sbq,
> > +				  int old_wake_index)
> >  {
> >  	int i, wake_index;
> > -
> > -	if (!atomic_read(&sbq->ws_active))
> > -		return NULL;
> > +	struct sbq_wait_state *ws;
> >  
> >  	wake_index = atomic_read(&sbq->wake_index);
> > -	for (i = 0; i < SBQ_WAIT_QUEUES; i++) {
> > -		struct sbq_wait_state *ws = &sbq->ws[wake_index];
> > +	if (old_wake_index != wake_index)
> > +		return;
> >  
> > +	for (i = 1; i < SBQ_WAIT_QUEUES; i++) {
> > +		wake_index = sbq_index_inc(wake_index);
> > +		ws = &sbq->ws[wake_index];
> > +		/* Find the next active waitqueue in round robin manner */
> >  		if (waitqueue_active(&ws->wait)) {
> > -			if (wake_index != atomic_read(&sbq->wake_index))
> > -				atomic_set(&sbq->wake_index, wake_index);
> > -			return ws;
> > +			atomic_cmpxchg(&sbq->wake_index, old_wake_index,
> > +				       wake_index);
> > +			return;
> >  		}
> > -
> > -		wake_index = sbq_index_inc(wake_index);
> >  	}
> > -
> > -	return NULL;
> >  }
> >  
> >  static bool __sbq_wake_up(struct sbitmap_queue *sbq)
> >  {
> >  	struct sbq_wait_state *ws;
> >  	unsigned int wake_batch;
> > -	int wait_cnt;
> > +	int wait_cnt, wake_index;
> >  
> > -	ws = sbq_wake_ptr(sbq);
> > -	if (!ws)
> > +	if (!atomic_read(&sbq->ws_active))
> >  		return false;
> >  
> > -	wait_cnt = atomic_dec_return(&ws->wait_cnt);
> > -	if (wait_cnt <= 0) {
> > -		int ret;
> > -
> > -		wake_batch = READ_ONCE(sbq->wake_batch);
> > -
> > -		/*
> > -		 * 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_index = atomic_read(&sbq->wake_index);
> > +	ws = &sbq->ws[wake_index];
> > +	/*
> > +	 * This can only happen in the first wakeup when sbitmap waitqueues
> > +	 * are no longer idle.
> > +	 */
> > +	if (!waitqueue_active(&ws->wait)) {
> > +		sbq_update_wake_index(sbq, wake_index);
> > +		return true;
> > +	}
> >  
> > -		/*
> > -		 * 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;
> > -		}
> > +	wait_cnt = atomic_dec_return(&ws->wait_cnt);
> > +	if (wait_cnt > 0)
> > +		return false;
> 
> The following race is still possible:
> 
> CPU1					CPU2
> __sbq_wake_up				__sbq_wake_up
>   wake_index = atomic_read(&sbq->wake_index);
> 					  wake_index = atomic_read(&sbq->wake_index);
> 
>   if (!waitqueue_active(&ws->wait)) -> not taken
> 					  if (!waitqueue_active(&ws->wait)) -> not taken
>   wait_cnt = atomic_dec_return(&ws->wait_cnt);
>   /* decremented to 0 now */
>   if (wait_cnt > 0) -> not taken
>   sbq_update_wake_index(sbq, wake_index);
>   if (wait_cnt < 0) -> not taken
>   ...
>   atomic_set(&ws->wait_cnt, wake_batch);
>   wake_up_nr(&ws->wait, wake_batch);
> 					  wait_cnt = atomic_dec_return(&ws->wait_cnt);
> 					  /*
> 					   * decremented to wake_batch - 1 but
> 					   * there are no tasks waiting anymore
> 					   * so the wakeup should have gone
> 					   * to a different waitqueue.
> 					   */
> 
> I have an idea how to fix all these lost wakeups, I'll try to code it
> whether it would look usable...

Thinking a bit more about it your code would just need a small tweak like:

	wait_cnt = atomic_dec_return(&ws->wait_cnt);
	/*
	 * Concurrent callers should call this function again
	 * to wakeup a new batch on a different 'ws'.
	 */
	if (wait_cnt < 0 || !waitqueue_active(&ws->wait)) {
		sbq_update_wake_index(sbq, wake_index);
		return true;
	}
	if (wait_cnt > 0)
		return false;
	sbq_update_wake_index(sbq, wake_index);

	wake_batch = READ_ONCE(sbq->wake_batch);
	wake_up_nr(&ws->wait, wake_batch);
	/*
	 * Pairs with the memory barrier in sbitmap_queue_resize() to
	 * ensure that we see the batch size update before the wait
	 * count is reset.
	 *
	 * Also pairs with the implicit barrier between decrementing
	 * wait_cnt and checking for waitqueue_active() to make sure
	 * waitqueue_active() sees results of the wakeup if
	 * atomic_dec_return() has seen results of the atomic_set.
	 */
	smp_mb__before_atomic();
	atomic_set(&ws->wait_cnt, wake_batch);

								Honza

> > +	sbq_update_wake_index(sbq, wake_index);
> > +	/*
> > +	 * Concurrent callers should call this function again
> > +	 * to wakeup a new batch on a different 'ws'.
> > +	 */
> > +	if (wait_cnt < 0)
> >  		return true;
> > -	}
> > +
> > +	wake_batch = READ_ONCE(sbq->wake_batch);
> > +	/*
> > +	 * 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();
> > +	atomic_set(&ws->wait_cnt, wake_batch);
> > +	wake_up_nr(&ws->wait, wake_batch);
> >  
> >  	return false;
> >  }
> > -- 
> > 2.31.1
> > 
> -- 
> Jan Kara <jack@suse.com>
> SUSE Labs, CR
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR