Re: [PATCH 2/3] drm/scheduler: Don't call wait_event_killable for signaled process.

From: Oleg Nesterov <oleg@redhat.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: David.Panariti@amd.com, "Michel Dänzer" <michel@daenzer.net>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	"amd-gfx list" <amd-gfx@lists.freedesktop.org>,
	"Alex Deucher" <Alexander.Deucher@amd.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Christian König" <Christian.Koenig@amd.com>,
	"Eric Biederman" <ebiederm@xmission.com>
Subject: Re: [PATCH 2/3] drm/scheduler: Don't call wait_event_killable for signaled process.
Date: Wed, 25 Apr 2018 16:18:07 +0200	[thread overview]
Message-ID: <20180425141807.GA8212@redhat.com> (raw)
In-Reply-To: <CAKMK7uF7LumuFWw3Xezw+QYDEhz6pFrJueZqFvSnmmoynYqpgQ@mail.gmail.com>

On 04/25, Daniel Vetter wrote:
>
> On Wed, Apr 25, 2018 at 3:22 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> > On 04/24, Daniel Vetter wrote:
> >>
> >> wait_event_killabel doesn't check for fatal_signal_pending before calling
> >> schedule, so definitely has a nice race there.
> >
> > This is fine. See the signal_pending_state() check in __schedule().
> >
> > And this doesn't differ from wait_event_interruptible(), it too doesn't
> > check signal_pending(), we rely on schedule() which must not block if the
> > caller is signalled/killed.
> >
> > The problem is that it is not clear what should fatal_signal_pending() or
> > even signal_pending() mean after exit_signals().
>
> Uh, I was totally thrown off in all the wait_event* macros and somehow
> landed in the _locked variants, which all need to recheck before they
> drop the lock, for efficiency reasons. See do_wait_intr().

Just in case, note that do_wait_intr() has to check signal_pending() for
completely differerent reason. We need to return non-zero code to stop the
main loop in __wait_event_interruptible_locked(); unlike ___wait_event()
it doesn't check signal_pending() itself.

Oleg.

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