linux-kernel - Re: [PATCH 1/2 v2] io_uring: Fix race condition when sqp thread goes to sleep

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Date:   Wed, 23 Jun 2021 09:52:53 -0400
From:   Olivier Langlois <olivier@...llion01.com>
To:     Pavel Begunkov <asml.silence@...il.com>,
        Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2 v2] io_uring: Fix race condition when sqp thread
 goes to sleep

On Wed, 2021-06-23 at 00:03 +0100, Pavel Begunkov wrote:
> On 6/22/21 11:42 PM, Olivier Langlois wrote:
> > On Tue, 2021-06-22 at 18:37 -0400, Olivier Langlois wrote:
> > > On Tue, 2021-06-22 at 21:45 +0100, Pavel Begunkov wrote:
> > > 
> > > 
> > > I can do that if you want but considering that the function is
> > > inline
> > > and the race condition is a relatively rare occurence, is the
> > > cost
> > > coming with inline expansion really worth it in this case?
> > > > 
> > On hand, there is the inline expansion concern.
> > 
> > OTOH, the benefit of going with your suggestion is that completions
> > generally precedes new submissions so yes, it might be better that
> > way.
> > 
> > I'm really unsure about this. I'm just raising the concern and I'll
> > let
> > you make the final decision...
> 
> It seems it may actually loop infinitely until it gets a signal,
> so yes. And even if not, rare stalls are nasty, they will ruin
> some 9s of latency and hard to catch.
> 
> That part is quite cold anyway, would generate some extra cold
> instructions, meh
> 
I'm not 100% sure to see the infinite loop possibility but I guess that
with some badly placed preemptions, it could take few iterations before
entering the block:

		if (sqt_spin || !time_after(jiffies, timeout)) {

So I will go ahead with your suggestion.

I'll retest the new patch version (it should be a formality) and I'll
resend an update once done.

Greetings,