Re: [PATCH 2/3] drm/scheduler: Don't call wait_event_killable for signaled process.
From: Daniel Vetter
Date: Tue Apr 24 2018 - 17:40:38 EST
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@xxxxxxx>
> > > > ---
> > > > 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
> 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.
> > 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
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Software Engineer, Intel Corporation