linux-kernel - Re: [PATCH v4 06/21] drm/i915: Prepare to dynamic dma-buf locking specification

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Date:   Fri, 2 Sep 2022 13:31:56 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     "Ruhl, Michael J" <michael.j.ruhl@...el.com>,
        Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        "Vivi, Rodrigo" <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Thomas Hellström <thomas_os@...pmail.org>,
        Christian König <christian.koenig@....com>,
        Chris Wilson <chris@...is-wilson.co.uk>
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        Christian Gmeiner <christian.gmeiner@...il.com>
Subject: Re: [PATCH v4 06/21] drm/i915: Prepare to dynamic dma-buf locking
 specification

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.