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Date:   Fri, 2 Sep 2022 13:38:30 +0300
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Subject: Re: [PATCH v4 06/21] drm/i915: Prepare to dynamic dma-buf locking
 specification
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From:   Dmitry Osipenko <digetx@gmail.com>
To:     "Ruhl, Michael J" <michael.j.ruhl@intel.com>,
        Dmitry Osipenko <dmitry.osipenko@collabora.com>,
        Jani Nikula <jani.nikula@linux.intel.com>,
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        Chris Wilson <chris@chris-wilson.co.uk>
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02.09.2022 13:31, Dmitry Osipenko пишет:
> 01.09.2022 17:02, Ruhl, Michael J пишет:
> ...
>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
>>> @@ -331,7 +331,19 @@ static void __i915_gem_free_objects(struct
>>> drm_i915_private *i915,
>>> 			continue;
>>> 		}
>>>
>>> +		/*
>>> +		 * dma_buf_unmap_attachment() requires reservation to be
>>> +		 * locked. The imported GEM shouldn't share reservation lock,
>>> +		 * so it's safe to take the lock.
>>> +		 */
>>> +		if (obj->base.import_attach)
>>> +			i915_gem_object_lock(obj, NULL);
>>
>> There is a lot of stuff going here.  Taking the lock may be premature...
>>
>>> 		__i915_gem_object_pages_fini(obj);
>>
>> The i915_gem_dmabuf.c:i915_gem_object_put_pages_dmabuf is where
>> unmap_attachment is actually called, would it make more sense to make
>> do the locking there?
> 
> The __i915_gem_object_put_pages() is invoked with a held reservation
> lock, while freeing object is a special time when we know that GEM is
> unused.
> 
> The __i915_gem_free_objects() was taking the lock two weeks ago until
> the change made Chris Wilson [1] reached linux-next.
> 
> [1]
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=2826d447fbd60e6a05e53d5f918bceb8c04e315c
> 
> I don't think we can take the lock within
> i915_gem_object_put_pages_dmabuf(), it may/should deadlock other code paths.

On the other hand, we can check whether the GEM's refcount number is
zero in i915_gem_object_put_pages_dmabuf() and then take the lock if
it's zero.

Also, seems it should be possible just to bail out from
i915_gem_object_put_pages_dmabuf() if refcount=0. The further
drm_prime_gem_destroy() will take care of unmapping. Perhaps this could
be the best option, I'll give it a test.