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Message-ID: <20230828121239.78a180e6@collabora.com>
Date: Mon, 28 Aug 2023 12:12:39 +0200
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Dmitry Osipenko <dmitry.osipenko@...labora.com>
Cc: David Airlie <airlied@...il.com>,
Gerd Hoffmann <kraxel@...hat.com>,
Gurchetan Singh <gurchetansingh@...omium.org>,
Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
Maxime Ripard <mripard@...nel.org>,
Thomas Zimmermann <tzimmermann@...e.de>,
Christian König <christian.koenig@....com>,
Qiang Yu <yuq825@...il.com>,
Steven Price <steven.price@....com>,
Emma Anholt <emma@...olt.net>, Melissa Wen <mwen@...lia.com>,
Will Deacon <will@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Boqun Feng <boqun.feng@...il.com>,
Mark Rutland <mark.rutland@....com>,
dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
kernel@...labora.com, virtualization@...ts.linux-foundation.org,
intel-gfx@...ts.freedesktop.org
Subject: Re: [PATCH v15 17/23] drm/shmem-helper: Add and use
drm_gem_shmem_resv_assert_held() helper
On Sun, 27 Aug 2023 20:54:43 +0300
Dmitry Osipenko <dmitry.osipenko@...labora.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@...labora.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));
>
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