linux-kernel - Re: [PATCH] sbitmap: fix possible io hung due to lost wakeup

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Date:   Wed, 7 Sep 2022 08:13:40 -0600
From:   Keith Busch <kbusch@...nel.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Yu Kuai <yukuai1@...weicloud.com>, axboe@...nel.dk, osandov@...com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yukuai3@...wei.com, yi.zhang@...wei.com
Subject: Re: [PATCH] sbitmap: fix possible io hung due to lost wakeup

On Wed, Sep 07, 2022 at 12:23:18PM +0200, Jan Kara wrote:
> On Tue 06-09-22 15:27:51, Keith Busch wrote:
> > On Wed, Aug 03, 2022 at 08:15:04PM +0800, Yu Kuai wrote:
> > >  	wait_cnt = atomic_dec_return(&ws->wait_cnt);
> > > -	if (wait_cnt <= 0) {
> > > -		int ret;
> > > +	/*
> > > +	 * For concurrent callers of this, callers should call this function
> > > +	 * again to wakeup a new batch on a different 'ws'.
> > > +	 */
> > > +	if (wait_cnt < 0 || !waitqueue_active(&ws->wait))
> > > +		return true;
> > 
> > If wait_cnt is '0', but the waitqueue_active happens to be false due to racing
> > with add_wait_queue(), this returns true so the caller will retry.
> 
> Well, note that sbq_wake_ptr() called to obtain 'ws' did waitqueue_active()
> check. So !waitqueue_active() should really happen only if waiter was woken
> up by someone else or so. Not that it would matter much but I wanted to
> point it out.
> 
> > The next atomic_dec will set the current waitstate wait_cnt < 0, which
> > also forces an early return true. When does the wake up happen, or
> > wait_cnt and wait_index get updated in that case?
> 
> I guess your concern could be rephrased as: Who's going to ever set
> ws->wait_cnt to value > 0 if we ever exit with wait_cnt == 0 due to
> !waitqueue_active() condition?
> 
> And that is a good question and I think that's a bug in this patch. I think
> we need something like:
> 
> 	...
> 	/*
> 	 * 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;
> 	/*
> 	 * If we decremented queue without waiters, retry to avoid lost
> 	 * wakeups.
> 	 */
> 	if (wait_cnt > 0)
> 		return !waitqueue_active(&ws->wait);

I'm not sure about this part. We've already decremented, so the freed bit is
accounted for against the batch. Returning true here may double-count the freed
bit, right?
 
> 	/*
> 	 * When wait_cnt == 0, we have to be particularly careful as we are
> 	 * responsible to reset wait_cnt regardless whether we've actually
> 	 * woken up anybody. But in case we didn't wakeup anybody, we still
> 	 * need to retry.
> 	 */
> 	ret = !waitqueue_active(&ws->wait);
> 	wake_batch = READ_ONCE(sbq->wake_batch);
> 	/*
> 	 * Wake up first in case that concurrent callers decrease wait_cnt
> 	 * while waitqueue is empty.
> 	 */
> 	wake_up_nr(&ws->wait, wake_batch);
> 	...
> 
> 	return ret;
> 
> Does this fix your concern Keith?

Other than the above comment, this does appear to address the concern. Thanks!