Re: [PATCH v15 17/23] drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper

[Boris Brezillon <boris.brezillon@collabora.com>]

On Tue, 29 Aug 2023 05:34:23 +0300
Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:

> On 8/28/23 13:12, Boris Brezillon wrote:
> > On Sun, 27 Aug 2023 20:54:43 +0300
> > Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:
> >   
> >> In a preparation of adding drm-shmem memory shrinker, move all reservation
> >> locking lockdep checks to use new drm_gem_shmem_resv_assert_held() that
> >> will resolve spurious lockdep warning about wrong locking order vs
> >> fs_reclam code paths during freeing of shmem GEM, where lockdep isn't
> >> aware that it's impossible to have locking contention with the fs_reclam
> >> at this special time.
> >>
> >> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> >> ---
> >>  drivers/gpu/drm/drm_gem_shmem_helper.c | 37 +++++++++++++++++---------
> >>  1 file changed, 25 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> index d96fee3d6166..ca5da976aafa 100644
> >> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> @@ -128,6 +128,23 @@ struct drm_gem_shmem_object *drm_gem_shmem_create(struct drm_device *dev, size_t
> >>  }
> >>  EXPORT_SYMBOL_GPL(drm_gem_shmem_create);
> >>  
> >> +static void drm_gem_shmem_resv_assert_held(struct drm_gem_shmem_object *shmem)
> >> +{
> >> +	/*
> >> +	 * Destroying the object is a special case.. drm_gem_shmem_free()
> >> +	 * calls many things that WARN_ON if the obj lock is not held.  But
> >> +	 * acquiring the obj lock in drm_gem_shmem_free() can cause a locking
> >> +	 * order inversion between reservation_ww_class_mutex and fs_reclaim.
> >> +	 *
> >> +	 * This deadlock is not actually possible, because no one should
> >> +	 * be already holding the lock when drm_gem_shmem_free() is called.
> >> +	 * Unfortunately lockdep is not aware of this detail.  So when the
> >> +	 * refcount drops to zero, we pretend it is already locked.
> >> +	 */
> >> +	if (kref_read(&shmem->base.refcount))
> >> +		drm_gem_shmem_resv_assert_held(shmem);
> >> +}
> >> +
> >>  /**
> >>   * drm_gem_shmem_free - Free resources associated with a shmem GEM object
> >>   * @shmem: shmem GEM object to free
> >> @@ -142,8 +159,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
> >>  	if (obj->import_attach) {
> >>  		drm_prime_gem_destroy(obj, shmem->sgt);
> >>  	} else if (!shmem->imported_sgt) {
> >> -		dma_resv_lock(shmem->base.resv, NULL);
> >> -
> >>  		drm_WARN_ON(obj->dev, kref_read(&shmem->vmap_use_count));
> >>  
> >>  		if (shmem->sgt) {
> >> @@ -156,8 +171,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
> >>  			drm_gem_shmem_put_pages_locked(shmem);  
> > 
> > AFAICT, drm_gem_shmem_put_pages_locked() is the only function that's
> > called in the free path and would complain about resv-lock not being
> > held. I think I'd feel more comfortable if we were adding a
> > drm_gem_shmem_free_pages() function that did everything
> > drm_gem_shmem_put_pages_locked() does except for the lock_held() check
> > and the refcount dec, and have it called here (and in
> > drm_gem_shmem_put_pages_locked()). This way we can keep using
> > dma_resv_assert_held() instead of having our own variant.  
> 
> It's not only drm_gem_shmem_free_pages(), but any drm-shmem function
> that drivers may use in the GEM's freeing callback.
> 
> For example, panfrost_gem_free_object() may unpin shmem BO and then do
> drm_gem_shmem_free().

Is this really a valid use case? If the GEM refcount dropped to zero,
we should certainly not have pages_pin_count > 0 (thinking of vmap-ed
buffers that might disappear while kernel still has a pointer to the
CPU-mapped area). The only reason we have this
drm_gem_shmem_put_pages_locked() in drm_gem_shmem_free() is because of
this implicit ref hold by the sgt, and IMHO, we should be stricter and
check that pages_use_count == 1 when sgt != NULL and pages_use_count ==
0 otherwise.

I actually think it's a good thing to try and catch any attempt to call
functions trying lock the resv in a path they're not supposed to. At
least we can decide whether these actions are valid or not in this
context, and provide dedicated helpers for the free path if they are.