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

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Message-ID: <20180424214027.GG25142@phenom.ffwll.local>
Date:   Tue, 24 Apr 2018 23:40:27 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Andrey Grodzovsky <Andrey.Grodzovsky@....com>
Cc:     Michel Dänzer <michel@...nzer.net>,
        linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, David.Panariti@....com,
        oleg@...hat.com, ebiederm@...ssion.com, Alexander.Deucher@....com,
        akpm@...ux-foundation.org, Christian.Koenig@....com
Subject: Re: [PATCH 2/3] drm/scheduler: Don't call wait_event_killable for
 signaled process.

On Tue, Apr 24, 2018 at 05:02:40PM -0400, Andrey Grodzovsky wrote:
> 
> 
> On 04/24/2018 03:44 PM, Daniel Vetter wrote:
> > On Tue, Apr 24, 2018 at 05:46:52PM +0200, Michel Dänzer wrote:
> > > Adding the dri-devel list, since this is driver independent code.
> > > 
> > > 
> > > On 2018-04-24 05:30 PM, Andrey Grodzovsky wrote:
> > > > Avoid calling wait_event_killable when you are possibly being called
> > > > from get_signal routine since in that case you end up in a deadlock
> > > > where you are alreay blocked in singla processing any trying to wait
> > > Multiple typos here, "[...] already blocked in signal processing and [...]"?
> > > 
> > > 
> > > > on a new signal.
> > > > 
> > > > Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@....com>
> > > > ---
> > > >   drivers/gpu/drm/scheduler/gpu_scheduler.c | 5 +++--
> > > >   1 file changed, 3 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/scheduler/gpu_scheduler.c b/drivers/gpu/drm/scheduler/gpu_scheduler.c
> > > > index 088ff2b..09fd258 100644
> > > > --- a/drivers/gpu/drm/scheduler/gpu_scheduler.c
> > > > +++ b/drivers/gpu/drm/scheduler/gpu_scheduler.c
> > > > @@ -227,9 +227,10 @@ void drm_sched_entity_do_release(struct drm_gpu_scheduler *sched,
> > > >   		return;
> > > >   	/**
> > > >   	 * The client will not queue more IBs during this fini, consume existing
> > > > -	 * queued IBs or discard them on SIGKILL
> > > > +	 * queued IBs or discard them when in death signal state since
> > > > +	 * wait_event_killable can't receive signals in that state.
> > > >   	*/
> > > > -	if ((current->flags & PF_SIGNALED) && current->exit_code == SIGKILL)
> > > > +	if (current->flags & PF_SIGNALED)
> > You want fatal_signal_pending() here, instead of inventing your own broken
> > version.
> 
> I rely on current->flags & PF_SIGNALED because this being set from within
> get_signal,
> meaning I am within signal processing  in which case I want to avoid any
> signal based wait for that task,
> From what i see in the code, task_struct.pending.signal is being set for
> other threads in same
> group (zap_other_threads) or for other scenarios, those task are still able
> to receive signals
> so calling wait_event_killable there will not have problem.
> > > >   		entity->fini_status = -ERESTARTSYS;
> > > >   	else
> > > >   		entity->fini_status = wait_event_killable(sched->job_scheduled,
> > But really this smells like a bug in wait_event_killable, since
> > wait_event_interruptible does not suffer from the same bug. It will return
> > immediately when there's a signal pending.
> 
> Even when wait_event_interruptible is called as following -
> ...->do_signal->get_signal->....->wait_event_interruptible ?
> I haven't tried it but wait_event_interruptible is very much alike to
> wait_event_killable so I would assume it will also
> not be interrupted if called like that. (Will give it a try just out of
> curiosity anyway)

wait_event_killabel doesn't check for fatal_signal_pending before calling
schedule, so definitely has a nice race there.

But if you're sure that you really need to check PF_SIGNALED, then I'm
honestly not clear on what you're trying to pull off here. Your sparse
explanation of what happens isn't enough, since I have no idea how you can
get from get_signal() to the above wait_event_killable callsite.
-Daniel

> 
> Andrey
> 
> > 
> > I think this should be fixed in core code, not papered over in some
> > subsystem.
> > -Daniel
> > 
> > > 
> > > -- 
> > > Earthling Michel Dänzer               |               http://www.amd.com
> > > Libre software enthusiast             |             Mesa and X developer
> > > _______________________________________________
> > > dri-devel mailing list
> > > dri-devel@...ts.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> 
> _______________________________________________
> dri-devel mailing list
> dri-devel@...ts.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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