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

[Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>]

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@amd.com> writes:
>>>
>>>> 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@amd.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,
>>> It doesn't mean that.  Unless you are called by do_coredump (you
>>> aren't).
>> Looking in latest code here
>> https://elixir.bootlin.com/linux/v4.17-rc2/source/kernel/signal.c#L2449
>> 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.
> -Daniel

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.

Andrey
>
>> Andrey
>>
>>> 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;
>>>>>>>     	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)
>>> As PF_EXITING is set want_signal should fail and the signal state of the
>>> task should not be updatable by signals.
>>>
>>> Eric
>>>
>>>
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