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

From: Andrey Grodzovsky
Date: Wed Apr 25 2018 - 09:08:24 EST

On 04/25/2018 03:14 AM, Daniel Vetter wrote:
On Tue, Apr 24, 2018 at 05:37:08PM -0400, Andrey Grodzovsky wrote:

On 04/24/2018 05:21 PM, Eric W. Biederman wrote:
Andrey Grodzovsky <Andrey.Grodzovsky@xxxxxxx> writes:

On 04/24/2018 03:44 PM, Daniel Vetter wrote:
On Tue, Apr 24, 2018 at 05:46:52PM +0200, Michel DÃnzer 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,
* 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
I rely on current->flags & PF_SIGNALED because this being set from
within get_signal,
It doesn't mean that. Unless you are called by do_coredump (you
Looking in latest code here
i see that current->flags |= PF_SIGNALED; is out side of
if (sig_kernel_coredump(signr)) {...} scope
Ok I read some more about this, and I guess you go through process exit
and then eventually close. But I'm not sure.

The code in drm_sched_entity_fini also looks strange: You unpark the
scheduler thread before you remove all the IBs. At least from the comment
that doesn't sound like what you want to do.

I think it should be safe for the dying scheduler entity since before that (in drm_sched_entity_do_release) we set it's runqueue to NULL
so no new jobs will be dequeued form it by the scheduler thread.

But in general, PF_SIGNALED is really something deeply internal to the
core (used for some book-keeping and accounting). The drm scheduler is the
only thing looking at it, so smells like a layering violation. I suspect
(but without knowing what you're actually trying to achive here can't be
sure) you want to look at something else.

E.g. PF_EXITING seems to be used in a lot more places to cancel stuff
that's no longer relevant when a task exits, not PF_SIGNALED. There's the
TIF_MEMDIE flag if you're hacking around issues with the oom-killer.

This here on the other hand looks really fragile, and probably only does
what you want to do by accident.

Yes , that what Eric also said and in the V2 patches i will try to change PF_EXITING

Another issue is changing wait_event_killable to wait_event_timeout where I need to understand
what TO value is acceptable for all the drivers using the scheduler, or maybe it should come as a property
of drm_sched_entity.



The closing of files does not happen in do_coredump.
Which means you are being called from do_exit.
In fact you are being called after exit_files which closes
the files. The actual __fput processing happens in task_work_run.

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.
Excpet that you are geing called after from do_exit and after exit_files
which is after exit_signal. Which means that PF_EXITING has been set.
Which implies that the kernel signal handling machinery has already
started being torn down.

Not as much as I would like to happen at that point as we are still
left with some old CLONE_PTHREAD messes in the code that need to be
cleaned up.

Still given the fact you are task_work_run it is quite possible even
release_task has been run on that task before the f_op->release method
is called. So you simply can not count on signals working.

Which in practice leaves a timeout for ending your wait. That code can
legitimately be in a context that is neither interruptible nor killable.

entity->fini_status = -ERESTARTSYS;
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)
As PF_EXITING is set want_signal should fail and the signal state of the
task should not be updatable by signals.


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