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 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.

>  
>  		drm_WARN_ON(obj->dev, kref_read(&shmem->pages_use_count));
> -
> -		dma_resv_unlock(shmem->base.resv);
>  	}
>  
>  	drm_gem_object_release(obj);
> @@ -170,7 +183,7 @@ static int drm_gem_shmem_get_pages_locked(struct drm_gem_shmem_object *shmem)
>  	struct drm_gem_object *obj = &shmem->base;
>  	struct page **pages;
>  
> -	dma_resv_assert_held(shmem->base.resv);
> +	drm_gem_shmem_resv_assert_held(shmem);
>  
>  	if (kref_get_unless_zero(&shmem->pages_use_count))
>  		return 0;
> @@ -228,7 +241,7 @@ static void drm_gem_shmem_kref_release_pages(struct kref *kref)
>   */
>  void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
>  {
> -	dma_resv_assert_held(shmem->base.resv);
> +	drm_gem_shmem_resv_assert_held(shmem);
>  
>  	kref_put(&shmem->pages_use_count, drm_gem_shmem_kref_release_pages);
>  }
> @@ -252,7 +265,7 @@ static int drm_gem_shmem_pin_locked(struct drm_gem_shmem_object *shmem)
>  {
>  	int ret;
>  
> -	dma_resv_assert_held(shmem->base.resv);
> +	drm_gem_shmem_resv_assert_held(shmem);
>  
>  	if (kref_get_unless_zero(&shmem->pages_pin_count))
>  		return 0;
> @@ -276,7 +289,7 @@ static void drm_gem_shmem_kref_unpin_pages(struct kref *kref)
>  
>  static void drm_gem_shmem_unpin_locked(struct drm_gem_shmem_object *shmem)
>  {
> -	dma_resv_assert_held(shmem->base.resv);
> +	drm_gem_shmem_resv_assert_held(shmem);
>  
>  	kref_put(&shmem->pages_pin_count, drm_gem_shmem_kref_unpin_pages);
>  }
> @@ -357,7 +370,7 @@ int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem,
>  	} else {
>  		pgprot_t prot = PAGE_KERNEL;
>  
> -		dma_resv_assert_held(shmem->base.resv);
> +		drm_gem_shmem_resv_assert_held(shmem);
>  
>  		if (kref_get_unless_zero(&shmem->vmap_use_count)) {
>  			iosys_map_set_vaddr(map, shmem->vaddr);
> @@ -426,7 +439,7 @@ void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
>  	if (obj->import_attach) {
>  		dma_buf_vunmap(obj->import_attach->dmabuf, map);
>  	} else {
> -		dma_resv_assert_held(shmem->base.resv);
> +		drm_gem_shmem_resv_assert_held(shmem);
>  		kref_put(&shmem->vmap_use_count, drm_gem_shmem_kref_vunmap);
>  	}
>  
> @@ -462,7 +475,7 @@ drm_gem_shmem_create_with_handle(struct drm_file *file_priv,
>   */
>  int drm_gem_shmem_madvise_locked(struct drm_gem_shmem_object *shmem, int madv)
>  {
> -	dma_resv_assert_held(shmem->base.resv);
> +	drm_gem_shmem_resv_assert_held(shmem);
>  
>  	if (shmem->madv >= 0)
>  		shmem->madv = madv;
> @@ -478,7 +491,7 @@ void drm_gem_shmem_purge_locked(struct drm_gem_shmem_object *shmem)
>  	struct drm_gem_object *obj = &shmem->base;
>  	struct drm_device *dev = obj->dev;
>  
> -	dma_resv_assert_held(shmem->base.resv);
> +	drm_gem_shmem_resv_assert_held(shmem);
>  
>  	drm_WARN_ON(obj->dev, !drm_gem_shmem_is_purgeable(shmem));
>