linux-kernel - Re: [PATCH 3/5] mm, notifier: Catch sleeping/blocking for !blockable

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Message-ID: <20190815070249.GB7444@phenom.ffwll.local>
Date:   Thu, 15 Aug 2019 09:02:49 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Christian König <christian.koenig@....com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 3/5] mm, notifier: Catch sleeping/blocking for !blockable

On Wed, Aug 14, 2019 at 09:00:29PM -0300, Jason Gunthorpe wrote:
> On Wed, Aug 14, 2019 at 10:20:25PM +0200, Daniel Vetter wrote:
> > We need to make sure implementations don't cheat and don't have a
> > possible schedule/blocking point deeply burried where review can't
> > catch it.
> > 
> > I'm not sure whether this is the best way to make sure all the
> > might_sleep() callsites trigger, and it's a bit ugly in the code flow.
> > But it gets the job done.
> > 
> > Inspired by an i915 patch series which did exactly that, because the
> > rules haven't been entirely clear to us.
> 
> I thought lockdep already was able to detect:
> 
>  spin_lock()
>  might_sleep();
>  spin_unlock()
> 
> Am I mistaken? If yes, couldn't this patch just inject a dummy lockdep
> spinlock?

Hm ... assuming I didn't get lost in the maze I think might_sleep (well
___might_sleep) doesn't do any lockdep checking at all. And we want
might_sleep, since that catches a lot more than lockdep.

Maybe you mixed it up with the hard/softirq context stuff that lockdep
tracks and complains about if you get it wrong?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch