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

[Jan Kara]

On Mon 20-06-22 21:44:16, Yu Kuai wrote:
> 在 2022/06/20 20:48, Jan Kara 写道:
> > 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@xxxxxxxxxx>
> > > 
> > > 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...
> Hi, Jan
> 
> Thanks for the analysis, it's right this is possible.
> > 
> > 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;
> > 	}
> 
> I'm thinking that if the wait_queue is still active, this will decrease
> 'wait_cnt' in old waitqueue while 'wake_index' is already moved to next
> waitqueue. This really broke the design...

I agree this can happen and it is not ideal. On the other hand the wakeup
is not really lost, just effectively delayed until we select this waitqueue
again so it should not result in any hangs. And other ways to avoid the
race seem more expensive to me...

								Honza

> > 	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@xxxxxxxx>
> > > SUSE Labs, CR
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR