Re: [PATCH v19 15/30] drm/shmem-helper: Avoid lockdep warning when pages are released

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

On Fri,  5 Jan 2024 21:46:09 +0300
Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:

> All drivers will be moved to get/put pages explicitly and then the last
> put_pages() will be invoked during gem_free() time by some drivers.
> We can't touch reservation lock when GEM is freed because that will cause
> a spurious warning from lockdep when shrinker support will be added.
> Lockdep doesn't know that fs_reclaim isn't functioning for a freed object,
> and thus, can't deadlock. Release pages directly without taking reservation
> lock if GEM is freed and its refcount is zero.
> 
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>

> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index f5ed64f78648..c7357110ca76 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -242,6 +242,22 @@ void drm_gem_shmem_put_pages(struct drm_gem_shmem_object *shmem)
>  	if (refcount_dec_not_one(&shmem->pages_use_count))
>  		return;
>  
> +	/*
> +	 * Destroying the object is a special case because acquiring
> +	 * the obj lock 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 GEM is released.  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)) {
> +		if (refcount_dec_and_test(&shmem->pages_use_count))
> +			drm_gem_shmem_free_pages(shmem);
> +		return;
> +	}
> +
>  	dma_resv_lock(shmem->base.resv, NULL);
>  	drm_gem_shmem_put_pages_locked(shmem);
>  	dma_resv_unlock(shmem->base.resv);