linux-kernel - Re: [PATCH -next v4] blk-mq: fix tag

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Message-ID: <Yd7n7xA9ecF1/0DK@smile.fi.intel.com>
Date:   Wed, 12 Jan 2022 16:38:39 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     John Garry <john.garry@...wei.com>
Cc:     QiuLaibin <qiulaibin@...wei.com>, axboe@...nel.dk,
        ming.lei@...hat.com, martin.petersen@...cle.com, hare@...e.de,
        johannes.thumshirn@....com, bvanassche@....org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next v4] blk-mq: fix tag_get wait task can't be awakened

On Wed, Jan 12, 2022 at 12:51:13PM +0000, John Garry wrote:
> On 12/01/2022 12:30, Andy Shevchenko wrote:
> > > > > +		if (test_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags) ||
> > > > > +		    test_and_set_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags)) {
> > > > Whoever wrote this code did too much defensive programming, because the first
> > > > conditional doesn't make much sense here. Am I right?
> > > > 
> > > I think because this judgement is in the general IO process, there are also
> > > some performance considerations here.
> > I didn't buy this. Is there any better argument why you need redundant
> > test_bit() call?
> 
> I think that the idea is that test_bit() is fast and test_and_set_bit() is
> slow; as such, if we generally expect the bit to be set, then there is no
> need to do the slower test_and_set_bit() always.

It doesn't sound thought through solution, the bit can be flipped in between,
so what is this all about? Maybe missing proper serialization somewhere else?

-- 
With Best Regards,
Andy Shevchenko