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Subject: Re: [PATCH 3/3] drm/amdgpu: Switch to interrupted wait to recover
 from ring hang.
To:     "Eric W. Biederman" <ebiederm@xmission.com>,
        "Panariti, David" <David.Panariti@amd.com>
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        "Koenig, Christian" <Christian.Koenig@amd.com>,
        "oleg@redhat.com" <oleg@redhat.com>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>
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From:   Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Message-ID: <d5a1b82b-a9c2-e6e8-1957-aab8acac242c@amd.com>
Date:   Wed, 25 Apr 2018 13:17:40 -0400
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On 04/24/2018 12:30 PM, Eric W. Biederman wrote:
> "Panariti, David" <David.Panariti@amd.com> writes:
>
>> Andrey Grodzovsky <andrey.grodzovsky@amd.com> writes:
>>> Kind of dma_fence_wait_killable, except that we don't have such API
>>> (maybe worth adding ?)
>> Depends on how many places it would be called, or think it might be called.  Can always factor on the 2nd time it's needed.
>> Factoring, IMO, rarely hurts.  The factored function can easily be visited using `M-.' ;->
>>
>> Also, if the wait could be very long, would a log message, something like "xxx has run for Y seconds."  help?
>> I personally hate hanging w/no info.
> Ugh.  This loop appears susceptible to loosing wake ups.  There are
> races between when a wake-up happens, when we clear the sleeping state,
> and when we test the stat to see if we should stat awake.  So yes
> implementing a dma_fence_wait_killable that handles of all that
> correctly sounds like an very good idea.

I am not clear here - could you be more specific about what races will 
happen here, more bellow
>
> Eric
>
>
>>> If the ring is hanging for some reason allow to recover the waiting by sending fatal signal.
>>>
>>> Originally-by: David Panariti <David.Panariti@amd.com>
>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
>>> ---
>>>    drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c | 14 ++++++++++----
>>>    1 file changed, 10 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c
>>> index eb80edf..37a36af 100644
>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c
>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c
>>> @@ -421,10 +421,16 @@ int amdgpu_ctx_wait_prev_fence(struct amdgpu_ctx *ctx, unsigned ring_id)
>>>
>>>        if (other) {
>>>                signed long r;
>>> -             r = dma_fence_wait_timeout(other, false, MAX_SCHEDULE_TIMEOUT);
>>> -             if (r < 0) {
>>> -                     DRM_ERROR("Error (%ld) waiting for fence!\n", r);
>>> -                     return r;
>>> +
>>> +             while (true) {
>>> +                     if ((r = dma_fence_wait_timeout(other, true,
>>> +                                     MAX_SCHEDULE_TIMEOUT)) >= 0)
>>> +                             return 0;
>>> +

Do you mean that by the time I reach here some other thread from my 
group already might dequeued SIGKILL since it's a shared signal and 
hence fatal_signal_pending will return false ? Or are you talking about 
the dma_fence_wait_timeout implementation in dma_fence_default_wait with 
schedule_timeout ?

Andrey

>>> +                     if (fatal_signal_pending(current)) {
>>> +                             DRM_ERROR("Error (%ld) waiting for fence!\n", r);
>>> +                             return r;
>>> +                     }
>>>                }
>>>        }
>>>
>>> --
>>> 2.7.4
>>>
> Eric