linux-kernel - Re: [RFC][PATCH] dm: Remove dm_bufio_cond

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Date:   Fri, 23 Sep 2016 11:13:22 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Mikulas Patocka <mpatocka@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Alasdair Kergon <agk@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        dm-devel@...hat.com, Joe Thornber <ejt@...hat.com>
Subject: Re: [RFC][PATCH] dm: Remove dm_bufio_cond_resched()

On Fri, 23 Sep 2016, Peter Zijlstra wrote:
> On Fri, Sep 23, 2016 at 10:00:37AM +0200, Thomas Gleixner wrote:
> > On Fri, 23 Sep 2016, Peter Zijlstra wrote:
> > > It is, might_sleep() implies might_resched(). In fact, that's all what
> > > PREEMPT_VOLUNTARY is, make the might_sleep() debug test imply a resched
> > > point.
> > 
> > Grr, how intuitive - NOT!
> 
> No, it actually makes sense. Because you 'obviously' only call
> might_sleep() in contexts that should be able to sleep (if not, it'll
> holler). So they're already placed right for preemption.

I disagree. might_sleep() is commonly known as a debug mechanism and it
existed before the preemption stuff went in. So the easy way to sprinkle
preemption points into the kernel was to hijack might_sleep(). I know it's
historical, but that doesnt make it any more intuitive.

Thanks,

	tglx