linux-kernel - Re: [PATCH drm-misc-next v5 4/6] drm/gpuvm: track/lock/validate external/evicted objects

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Message-ID: <6489f31f-8929-3e59-fbef-a22049cccbe3@redhat.com>
Date:   Wed, 4 Oct 2023 14:57:08 +0200
From:   Danilo Krummrich <dakr@...hat.com>
To:     Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>, airlied@...il.com,
        daniel@...ll.ch, matthew.brost@...el.com, sarah.walker@...tec.com,
        donald.robson@...tec.com, boris.brezillon@...labora.com,
        christian.koenig@....com, faith@...strand.net
Cc:     dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH drm-misc-next v5 4/6] drm/gpuvm: track/lock/validate
 external/evicted objects

On 10/3/23 11:11, Thomas Hellström wrote:

<snip>

>>> +
>>> +/**
>>> + * drm_gpuvm_bo_evict() - add / remove a &drm_gpuvm_bo to / from the &drm_gpuvms
>>> + * evicted list
>>> + * @vm_bo: the &drm_gpuvm_bo to add or remove
>>> + * @evict: indicates whether the object is evicted
>>> + *
>>> + * Adds a &drm_gpuvm_bo to or removes it from the &drm_gpuvms evicted list.
>>> + */
>>> +void
>>> +drm_gpuvm_bo_evict(struct drm_gpuvm_bo *vm_bo, bool evict)
>>> +{
>>> +    struct drm_gem_object *obj = vm_bo->obj;
>>> +
>>> +    dma_resv_assert_held(obj->resv);
>>> +
>>> +    /* Always lock list transactions, even if DRM_GPUVM_RESV_PROTECTED is
>>> +     * set. This is required to protect multiple concurrent calls to
>>> +     * drm_gpuvm_bo_evict() with BOs with different dma_resv.
>>> +     */
>>
>> This doesn't work. The RESV_PROTECTED case requires the evicted flag we discussed before. The list is either protected by the spinlock or the resv. Otherwise a list add could race with a list removal elsewhere.

I think it does unless I miss something, but it might be a bit subtle though.

Concurrent drm_gpuvm_bo_evict() are protected by the spinlock. Additionally, when
drm_gpuvm_bo_evict() is called we hold the dma-resv of the corresponding GEM object.

In drm_gpuvm_validate() I assert that we hold *all* dma-resv, which implies that no
one can call drm_gpuvm_bo_evict() on any of the VM's objects and no one can add a new
one and directly call drm_gpuvm_bo_evict() on it either.

>>
>> Thanks,
>>
>> Thomas
>>
>>
>