Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.35 as permitted sender) client-ip=23.128.96.35;
Message-ID: <6489f31f-8929-3e59-fbef-a22049cccbe3@redhat.com>
Date:   Wed, 4 Oct 2023 14:57:08 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.13.0
From:   Danilo Krummrich <dakr@redhat.com>
Subject: Re: [PATCH drm-misc-next v5 4/6] drm/gpuvm: track/lock/validate
 external/evicted objects
To:     =?UTF-8?Q?Thomas_Hellstr=c3=b6m?= 
        <thomas.hellstrom@linux.intel.com>, airlied@gmail.com,
        daniel@ffwll.ch, matthew.brost@intel.com, sarah.walker@imgtec.com,
        donald.robson@imgtec.com, boris.brezillon@collabora.com,
        christian.koenig@amd.com, faith@gfxstrand.net
Cc:     dri-devel@lists.freedesktop.org, nouveau@lists.freedesktop.org,
        linux-kernel@vger.kernel.org
References: <20230928191624.13703-1-dakr@redhat.com>
 <20230928191624.13703-5-dakr@redhat.com>
 <e4e68970-c7c9-55e2-9483-01252f38c956@linux.intel.com>
 <6b16ab6f-b1a2-efdb-04bf-5af4c3de381b@linux.intel.com>
Content-Language: en-US
Organization: RedHat
In-Reply-To: <6b16ab6f-b1a2-efdb-04bf-5af4c3de381b@linux.intel.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk

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
>>
>>
>