Re: [PATCH v3 0/2] drm: Fix dma_resv deadlock at drm object pin time

Thomas Zimmermann
2024-05-02T12:18:21Z

Hi

Am 02.05.24 um 14:00 schrieb Boris Brezillon:
> On Thu, 2 May 2024 13:59:41 +0200
> Boris Brezillon <boris.brezillon@collabora.com> wrote:
>
>> Hi Thomas,
>>
>> On Thu, 2 May 2024 13:51:16 +0200
>> Thomas Zimmermann <tzimmermann@suse.de> wrote:
>>
>>> Hi,
>>>
>>> ignoring my r-b on patch 1, I'd like to rethink the current patches in
>>> general.
>>>
>>> I think drm_gem_shmem_pin() should become the locked version of _pin(),
>>> so that drm_gem_shmem_object_pin() can call it directly. The existing
>>> _pin_unlocked() would not be needed any longer. Same for the _unpin()
>>> functions. This change would also fix the consistency with the semantics
>>> of the shmem _vmap() functions, which never take reservation locks.
>>>
>>> There are only two external callers of drm_gem_shmem_pin(): the test
>>> case and panthor. These assume that drm_gem_shmem_pin() acquires the
>>> reservation lock. The test case should likely call drm_gem_pin()
>>> instead. That would acquire the reservation lock and the test would
>>> validate that shmem's pin helper integrates well into the overall GEM
>>> framework. The way panthor uses drm_gem_shmem_pin() looks wrong to me.
>>> For now, it could receive a wrapper that takes the lock and that's it.
>> I do agree that the current inconsistencies in the naming is
>> troublesome (sometimes _unlocked, sometimes _locked, with the version
>> without any suffix meaning either _locked or _unlocked depending on
>> what the suffixed version does), and that's the very reason I asked
>> Dmitry to address that in his shrinker series [1]. So, ideally I'd
>> prefer if patches from Dmitry's series were applied instead of
>> trying to fix that here (IIRC, we had an ack from Maxime).
> With the link this time :-).
>
> [1]https://lore.kernel.org/lkml/20240105184624.508603-1-dmitry.osipenko@collabora.com/T/

Thanks. I remember these patches. Somehow I thought they would have been
merged already. I wasn't super happy about the naming changes in patch
5, because the names of the GEM object callbacks do no longer correspond
with their implementations. But anyway.

If we go that direction, we should here simply push drm_gem_shmem_pin()
and drm_gem_shmem_unpin() into panthor and update the shmem tests with
drm_gem_pin(). Panfrost and lima would call drm_gem_shmem_pin_locked().
IMHO we should not promote the use of drm_gem_shmem_object_*()
functions, as they are meant to be callbacks for struct
drm_gem_object_funcs. (Auto-generating them would be nice.)

Best regards
Thomas

>
>> Regards,
>>
>> Boris

--
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Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
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