Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH 3/3] drm/amdgpu: Switch to interrupted wait to recover
 from ring hang.
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     "Panariti, David" <David.Panariti@amd.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "amd-gfx@lists.freedesktop.org" <amd-gfx@lists.freedesktop.org>,
        "Deucher, Alexander" <Alexander.Deucher@amd.com>,
        "Koenig, Christian" <Christian.Koenig@amd.com>,
        "oleg@redhat.com" <oleg@redhat.com>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>
References: <1524583836-12130-1-git-send-email-andrey.grodzovsky@amd.com>
 <1524583836-12130-4-git-send-email-andrey.grodzovsky@amd.com>
 <DM5PR12MB244017F98FC732EB5DD86E0395880@DM5PR12MB2440.namprd12.prod.outlook.com>
 <b4309ea4-c1a0-5811-040b-4390ce6f297f@amd.com>
 <DM5PR12MB24408FF7C780D95307FB2BAD95880@DM5PR12MB2440.namprd12.prod.outlook.com>
 <87bme8bm9g.fsf@xmission.com> <d5a1b82b-a9c2-e6e8-1957-aab8acac242c@amd.com>
 <87h8nzt39f.fsf@xmission.com>
From:   Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Message-ID: <7ea85f74-a52b-0e10-63ff-c0ccf2f63d8b@amd.com>
Date:   Thu, 26 Apr 2018 08:28:53 -0400
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.4.0
MIME-Version: 1.0
In-Reply-To: <87h8nzt39f.fsf@xmission.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Content-Language: en-US
Received-SPF: None (protection.outlook.com: amd.com does not designate
 permitted sender hosts)
SpamDiagnosticOutput: 1:99
SpamDiagnosticMetadata: NSPM
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 26 Apr 2018 12:28:58.7586 (UTC)
X-MS-Exchange-CrossTenant-Network-Message-Id: 164f5848-c6eb-4eb0-b8db-08d5ab71499e
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d
X-MS-Exchange-Transport-CrossTenantHeadersStamped: MWHPR1201MB0128
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


On 04/25/2018 04:55 PM, Eric W. Biederman wrote:
> Andrey Grodzovsky <Andrey.Grodzovsky@amd.com> writes:
>
>> 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 ?
> Given Oleg's earlier comment about the scheduler having special cases
> for signals I might be wrong.  But in general there is a pattern:
>
> 	for (;;) {
> 		set_current_state(TASK_UNINTERRUPTIBLE);
> 		if (loop_is_done())
> 			break;
> 		schedule();
> 	}
>          set_current_state(TASK_RUNNING);
>
> If you violate that pattern by testing for a condition without
> having first set your task as TASK_UNINTERRUPTIBLE (or whatever your
> sleep state is).  Then it is possible to miss a wake-up that
> tests the condidtion.
>
> Thus I am quite concerned that there is a subtle corner case where
> you can miss a wakeup and not retest fatal_signal_pending().


I see the general problem now. In this particular case 
dma_fence_default_wait
and the caller of wake_up_state use lock for protecting wake up delivery 
and wakeup condition
and also dma_fence_default_wait retests the wakeup condition on entry.
But obviously it's a bad practice to rely on API's internal 
implementation for assumptions
in client code.

>
> Given that there is is a timeout the worst case might have you sleep
> MAX_SCHEDULE_TIMEOUT instead of indefinitely.

It actually means never wake

>
> Without a comment why this is safe, or having fatal_signal_pending
> check integrated into dma_fence_wait_timeout I am not comfortable
> with this loop.

Agree, fatal_signal_pending should be part of the wait function.

Andrey

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