Re: [Intel-gfx] ✗ Fi.CI.BAT: failure for drm/i915: implement internal workqueues (rev3)

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 06/06/2023 12:06, Coelho, Luciano wrote:
> On Tue, 2023-06-06 at 11:06 +0100, Tvrtko Ursulin wrote:
>> On 05/06/2023 16:06, Jani Nikula wrote:
>>> On Wed, 31 May 2023, Patchwork <patchwork@emeril.freedesktop.org> wrote:
>>>> #### Possible regressions ####
>>>>
>>>>     * igt@gem_close_race@basic-process:
>>>>       - fi-blb-e6850:       [PASS][1] -> [ABORT][2]
>>>>      [1]: https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_13203/fi-blb-e6850/igt@gem_close_race@basic-process.html
>>>>      [2]: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_117618v3/fi-blb-e6850/igt@gem_close_race@basic-process.html
>>>>       - fi-hsw-4770:        [PASS][3] -> [ABORT][4]
>>>>      [3]: https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_13203/fi-hsw-4770/igt@gem_close_race@basic-process.html
>>>>      [4]: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_117618v3/fi-hsw-4770/igt@gem_close_race@basic-process.html
>>>>       - fi-elk-e7500:       [PASS][5] -> [ABORT][6]
>>>>      [5]: https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_13203/fi-elk-e7500/igt@gem_close_race@basic-process.html
>>>>      [6]: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_117618v3/fi-elk-e7500/igt@gem_close_race@basic-process.html
>>>>
>>>>     * igt@i915_selftest@live@evict:
>>>>       - bat-adlp-9:         [PASS][7] -> [ABORT][8]
>>>>      [7]: https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_13203/bat-adlp-9/igt@i915_selftest@live@evict.html
>>>>      [8]: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_117618v3/bat-adlp-9/igt@i915_selftest@live@evict.html
>>>>       - bat-rpls-2:         [PASS][9] -> [ABORT][10]
>>>>      [9]: https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_13203/bat-rpls-2/igt@i915_selftest@live@evict.html
>>>>      [10]: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_117618v3/bat-rpls-2/igt@i915_selftest@live@evict.html
>>>>       - bat-adlm-1:         [PASS][11] -> [ABORT][12]
>>>>      [11]: https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_13203/bat-adlm-1/igt@i915_selftest@live@evict.html
>>>>      [12]: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_117618v3/bat-adlm-1/igt@i915_selftest@live@evict.html
>>>>       - bat-rpls-1:         [PASS][13] -> [ABORT][14]
>>>>      [13]: https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_13203/bat-rpls-1/igt@i915_selftest@live@evict.html
>>>>      [14]: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_117618v3/bat-rpls-1/igt@i915_selftest@live@evict.html
>>>
>>> This still fails consistently, I have no clue why, and the above aren't
>>> even remotely related to display.
>>>
>>> What now? Tvrtko?
>>
>> Hmm..
>>
>> <4> [46.782321] Chain exists of:
>>     (wq_completion)i915 --> (work_completion)(&i915->mm.free_work) --> &vm->mutex
>> <4> [46.782329]  Possible unsafe locking scenario:
>> <4> [46.782332]        CPU0                    CPU1
>> <4> [46.782334]        ----                    ----
>> <4> [46.782337]   lock(&vm->mutex);
>> <4> [46.782340]                                lock((work_completion)(&i915->mm.free_work));
>> <4> [46.782344]                                lock(&vm->mutex);
>> <4> [46.782348]   lock((wq_completion)i915);
>>
>>
>> "(wq_completion)i915"
>>
>> So it's not about the new wq even. Perhaps it is this hunk:
>>
>> --- a/drivers/gpu/drm/i915/intel_wakeref.c
>> +++ b/drivers/gpu/drm/i915/intel_wakeref.c
>> @@ -75,7 +75,7 @@ void __intel_wakeref_put_last(struct intel_wakeref *wf, unsigned long flags)
>>    
>>    	/* Assume we are not in process context and so cannot sleep. */
>>    	if (flags & INTEL_WAKEREF_PUT_ASYNC || !mutex_trylock(&wf->mutex)) {
>> -		mod_delayed_work(system_wq, &wf->work,
>> +		mod_delayed_work(wf->i915->wq, &wf->work,
>>
>> Transformation from this patch otherwise is system_wq with the new unordered wq, so I'd try that first.
> 
> Indeed this seems to be exactly the block that is causing the issue.  I
> was sort of bisecting through all these changes and reverting this one
> prevents the lockdep splat from happening...
> 
> So there's something that needs to be synced with the system_wq here,
> but what? I need to dig into it.

AFAICT it is saying that i915->mm.free_work and engine->wakeref.work 
must not be on the same ordered wq. Otherwise execbuf call trace 
flushing under vm->mutex can deadlock against the free worker trying to 
grab vm->mutex. If engine->wakeref.work is on a separate unordered wq it 
would be safe since then execution will not be serialized with the 
free_work. So just using the new i915->unordered_wq for this hunk should 
work.

Regards,

Tvrtko