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

Dmitry Osipenko
2022-04-28T18:31:14Z

Hello Daniel,

27.04.2022 17:50, Daniel Vetter пишет:
> On Mon, Apr 18, 2022 at 10:18:54PM +0300, Dmitry Osipenko wrote:
>> 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.
>
> Apologies for the late reply, I'm flooded.
>
> I discussed this with Daniel Stone last week in a chat, roughly what we
> need to do is:
>
> 1. Pick a function from shmem helpers.
>
> 2. Go through all drivers that call this, and make sure that we acquire
> dma_resv_lock in the top level driver entry point for this.
>
> 3. Once all driver code paths are converted, add a dma_resv_assert_held()
> call to that function to make sure you have it all correctly.
> 4. Repeate 1-3 until all shmem helper functions are converted over.
Somehow I didn't notice the existence of dma_resv_assert_held(), thank
you for the suggestion :)

>
> 5. Ditch the 3 different shmem helper locks.
>
> The trouble is that I forgot that vmap is a thing, so that needs more
> work. I think there's two approaches here:
> - Do the vmap at import time. This is the trick we used to untangle the
> dma_resv_lock issues around dma_buf_attachment_map()

> - Change the dma_buf_vmap rules that callers must hold the dma_resv_lock.

I'll consider this option for v6, thank you.

I see now that you actually want to define the new rules for the
dma-bufs in general and not only in the context of the DRM code, this
now makes much more sense to me.

> - Maybe also do what you suggest and keep a separate lock for this, but
> the fundamental issue is that this doesn't really work - if you share
> buffers both ways with two drivers using shmem helpers, then the
> ordering of this vmap_count_mutex vs dma_resv_lock is inconsistent and
> you can get some nice deadlocks. So not a great approach (and also the
> reason why we really need to get everyone to move towards dma_resv_lock
> as _the_ buffer object lock, since otherwise we'll never get a
> consistent lock nesting hierarchy).

The separate locks should work okay because it will be always the
exporter that takes the dma_resv_lock. But I agree that it's less ideal
than defining the new rules for dma-bufs since sometime you will take
the resv lock and sometime not, potentially hiding bugs related to lockings.

> The trouble here is that trying to be clever and doing the conversion just
> in shmem helpers wont work, because there's a lot of cases where the
> drivers are all kinds of inconsistent with their locking.
>
> Adding Daniel S, also maybe for questions it'd be fastest to chat on irc?

My nickname is digetx on the #dri-devel channel, feel free to ping me if
needed. Right now yours suggestions are clear to me, hence no extra
questions.

Thank you for the review.