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Date:   Wed, 7 Sep 2022 18:41:50 +0200
From:   Jan Kara <jack@suse.cz>
To:     Keith Busch <kbusch@kernel.org>
Cc:     Jan Kara <jack@suse.cz>, Yu Kuai <yukuai1@huaweicloud.com>,
        axboe@kernel.dk, osandov@fb.com, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, yukuai3@huawei.com,
        yi.zhang@huawei.com
Subject: Re: [PATCH] sbitmap: fix possible io hung due to lost wakeup
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On Wed 07-09-22 08:13:40, Keith Busch wrote:
> 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?

Yes, we may wake up waiters unnecessarily frequently. But that's a
performance issue at worst and only if it happens frequently. So I don't
think it matters in practice (famous last words ;).

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