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

[Daniel Vetter <daniel@ffwll.ch>]

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.

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.
- 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 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?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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