linux-kernel - Re: [PATCH 2/3] drm/scheduler: Don't call wait_event

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Date:   Tue, 24 Apr 2018 17:37:08 -0400
From:   Andrey Grodzovsky <Andrey.Grodzovsky@....com>
To:     "Eric W. Biederman" <ebiederm@...ssion.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, 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 04/24/2018 05:21 PM, Eric W. Biederman wrote:
> Andrey Grodzovsky <Andrey.Grodzovsky@....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@....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

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
>
>