Re: [PATCH v4 10/15] drm/shmem-helper: Take reservation lock instead of drm_gem_shmem locks

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

Hello,

On 4/18/22 21:38, Thomas Zimmermann wrote:
> Hi
> 
> Am 18.04.22 um 00:37 schrieb Dmitry Osipenko:
>> Replace drm_gem_shmem locks with the reservation lock to make GEM
>> lockings more consistent.
>>
>> Previously drm_gem_shmem_vmap() and drm_gem_shmem_get_pages() were
>> protected by separate locks, now it's the same lock, but it doesn't
>> make any difference for the current GEM SHMEM users. Only Panfrost
>> and Lima drivers use vmap() and they do it in the slow code paths,
>> hence there was no practical justification for the usage of separate
>> lock in the vmap().
>>
>> Suggested-by: Daniel Vetter <daniel@ffwll.ch>
>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
>> ---
...
>>   @@ -310,7 +306,7 @@ static int drm_gem_shmem_vmap_locked(struct
>> drm_gem_shmem_object *shmem,
>>       } else {
>>           pgprot_t prot = PAGE_KERNEL;
>>   -        ret = drm_gem_shmem_get_pages(shmem);
>> +        ret = drm_gem_shmem_get_pages_locked(shmem);
>>           if (ret)
>>               goto err_zero_use;
>>   @@ -360,11 +356,11 @@ int drm_gem_shmem_vmap(struct
>> drm_gem_shmem_object *shmem,
>>   {
>>       int ret;
>>   -    ret = mutex_lock_interruptible(&shmem->vmap_lock);
>> +    ret = dma_resv_lock_interruptible(shmem->base.resv, NULL);
>>       if (ret)
>>           return ret;
>>       ret = drm_gem_shmem_vmap_locked(shmem, map);
> 
> Within drm_gem_shmem_vmap_locked(), there's a call to dma_buf_vmap() for
> imported pages. If the exporter side also holds/acquires the same
> reservation lock as our object, the whole thing can deadlock. We cannot
> move dma_buf_vmap() out of the CS, because we still need to increment
> the reference counter. I honestly don't know how to easily fix this
> problem. There's a TODO item about replacing these locks at [1]. As
> Daniel suggested this patch, we should talk to him about the issue.
> 
> Best regards
> Thomas
> 
> [1]
> https://www.kernel.org/doc/html/latest/gpu/todo.html#move-buffer-object-locking-to-dma-resv-lock

Indeed, good catch! Perhaps we could simply use a separate lock for the
vmapping of the *imported* GEMs? The vmap_use_count is used only by
vmap/vunmap, so it doesn't matter which lock is used by these functions
in the case of imported GEMs since we only need to protect the
vmap_use_count.