[Intel-gfx] [PATCH v5 20/20] dma-resv: Give the docs a do-over

Daniel Vetter
2021-08-05T10:48:10Z

Specifically document the new/clarified rules around how the shared
fences do not have any ordering requirements against the exclusive
fence.

But also document all the things a bit better, given how central
struct dma_resv to dynamic buffer management the docs have been very
inadequat.

- Lots more links to other pieces of the puzzle. Unfortunately
ttm_buffer_object has no docs, so no links :-(

- Explain/complain a bit about dma_resv_locking_ctx(). I still don't
like that one, but fixing the ttm call chains is going to be
horrible. Plus we want to plug in real slowpath locking when we do
that anyway.

- Main part of the patch is some actual docs for struct dma_resv.

Overall I think we still have a lot of bad naming in this area (e.g.
dma_resv.fence is singular, but contains the multiple shared fences),
but I think that's more indicative of how the semantics and rules are
just not great.

Another thing that's real awkard is how chaining exclusive fences
right now means direct dma_resv.exclusive_fence pointer access with an
rcu_assign_pointer. Not so great either.

v2:
- Fix a pile of typos (Matt, Jason)
- Hammer it in that breaking the rules leads to use-after-free issues
around dma-buf sharing (Christian)

Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
---
drivers/dma-buf/dma-resv.c | 24 ++++++---
include/linux/dma-buf.h | 7 +++
include/linux/dma-resv.h | 104 +++++++++++++++++++++++++++++++++++--
3 files changed, 124 insertions(+), 11 deletions(-)

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index e744fd87c63c..84fbe60629e3 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -48,6 +48,8 @@
* write operations) or N shared fences (read operations). The RCU
* mechanism is used to protect read access to fences from locked
* write-side updates.
+ *
+ * See struct dma_resv for more details.
*/

DEFINE_WD_CLASS(reservation_ww_class);
@@ -137,7 +139,11 @@ EXPORT_SYMBOL(dma_resv_fini);
* @num_fences: number of fences we want to add
*
* Should be called before dma_resv_add_shared_fence(). Must
- * be called with obj->lock held.
+ * be called with @obj locked through dma_resv_lock().
+ *
+ * Note that the preallocated slots need to be re-reserved if @obj is unlocked
+ * at any time before calling dma_resv_add_shared_fence(). This is validated
+ * when CONFIG_DEBUG_MUTEXES is enabled.
*
* RETURNS
* Zero for success, or -errno
@@ -234,8 +240,10 @@ EXPORT_SYMBOL(dma_resv_reset_shared_max);
* @obj: the reservation object
* @fence: the shared fence to add
*
- * Add a fence to a shared slot, obj->lock must be held, and
+ * Add a fence to a shared slot, @obj must be locked with dma_resv_lock(), and
* dma_resv_reserve_shared() has been called.
+ *
+ * See also &dma_resv.fence for a discussion of the semantics.
*/
void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
{
@@ -278,9 +286,11 @@ EXPORT_SYMBOL(dma_resv_add_shared_fence);
/**
* dma_resv_add_excl_fence - Add an exclusive fence.
* @obj: the reservation object
- * @fence: the shared fence to add
+ * @fence: the exclusive fence to add
*
- * Add a fence to the exclusive slot. The obj->lock must be held.
+ * Add a fence to the exclusive slot. @obj must be locked with dma_resv_lock().
+ * Note that this function replaces all fences attached to @obj, see also
+ * &dma_resv.fence_excl for a discussion of the semantics.
*/
void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
{
@@ -609,9 +619,11 @@ static inline int dma_resv_test_signaled_single(struct dma_fence *passed_fence)
* fence
*
* Callers are not required to hold specific locks, but maybe hold
- * dma_resv_lock() already
+ * dma_resv_lock() already.
+ *
* RETURNS
- * true if all fences signaled, else false
+ *
+ * True if all fences signaled, else false.
*/
bool dma_resv_test_signaled(struct dma_resv *obj, bool test_all)
{
diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
index 678b2006be78..fc62b5f9980c 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -420,6 +420,13 @@ struct dma_buf {
* - Dynamic importers should set fences for any access that they can't
* disable immediately from their &dma_buf_attach_ops.move_notify
* callback.
+ *
+ * IMPORTANT:
+ *
+ * All drivers must obey the struct dma_resv rules, specifically the
+ * rules for updating fences, see &dma_resv.fence_excl and
+ * &dma_resv.fence. If these dependency rules are broken access tracking
+ * can be lost resulting in use after free issues.
*/
struct dma_resv *resv;

diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index e1ca2080a1ff..9100dd3dc21f 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -62,16 +62,90 @@ struct dma_resv_list {

/**
* struct dma_resv - a reservation object manages fences for a buffer
- * @lock: update side lock
- * @seq: sequence count for managing RCU read-side synchronization
- * @fence_excl: the exclusive fence, if there is one currently
- * @fence: list of current shared fences
+ *
+ * There are multiple uses for this, with sometimes slightly different rules in
+ * how the fence slots are used.
+ *
+ * One use is to synchronize cross-driver access to a struct dma_buf, either for
+ * dynamic buffer management or just to handle implicit synchronization between
+ * different users of the buffer in userspace. See &dma_buf.resv for a more
+ * in-depth discussion.
+ *
+ * The other major use is to manage access and locking within a driver in a
+ * buffer based memory manager. struct ttm_buffer_object is the canonical
+ * example here, since this is where reservation objects originated from. But
+ * use in drivers is spreading and some drivers also manage struct
+ * drm_gem_object with the same scheme.
*/
struct dma_resv {
+ /**
+ * @lock:
+ *
+ * Update side lock. Don't use directly, instead use the wrapper
+ * functions like dma_resv_lock() and dma_resv_unlock().
+ *
+ * Drivers which use the reservation object to manage memory dynamically
+ * also use this lock to protect buffer object state like placement,
+ * allocation policies or throughout command submission.
+ */
struct ww_mutex lock;
+
+ /**
+ * @seq:
+ *
+ * Sequence count for managing RCU read-side synchronization, allows
+ * read-only access to @fence_excl and @fence while ensuring we take a
+ * consistent snapshot.
+ */
seqcount_ww_mutex_t seq;

+ /**
+ * @fence_excl:
+ *
+ * The exclusive fence, if there is one currently.
+ *
+ * There are two ways to update this fence:
+ *
+ * - First by calling dma_resv_add_excl_fence(), which replaces all
+ * fences attached to the reservation object. To guarantee that no
+ * fences are lost, this new fence must signal only after all previous
+ * fences, both shared and exclusive, have signalled. In some cases it
+ * is convenient to achieve that by attaching a struct dma_fence_array
+ * with all the new and old fences.
+ *
+ * - Alternatively the fence can be set directly, which leaves the
+ * shared fences unchanged. To guarantee that no fences are lost, this
+ * new fence must signal only after the previous exclusive fence has
+ * signalled. Since the shared fences are staying intact, it is not
+ * necessary to maintain any ordering against those. If semantically
+ * only a new access is added without actually treating the previous
+ * one as a dependency the exclusive fences can be strung together
+ * using struct dma_fence_chain.
+ *
+ * Note that actual semantics of what an exclusive or shared fence mean
+ * is defined by the user, for reservation objects shared across drivers
+ * see &dma_buf.resv.
+ */
struct dma_fence __rcu *fence_excl;
+
+ /**
+ * @fence:
+ *
+ * List of current shared fences.
+ *
+ * There are no ordering constraints of shared fences against the
+ * exclusive fence slot. If a waiter needs to wait for all access, it
+ * has to wait for both sets of fences to signal.
+ *
+ * A new fence is added by calling dma_resv_add_shared_fence(). Since
+ * this often needs to be done past the point of no return in command
+ * submission it cannot fail, and therefore sufficient slots need to be
+ * reserved by calling dma_resv_reserve_shared().
+ *
+ * Note that actual semantics of what an exclusive or shared fence mean
+ * is defined by the user, for reservation objects shared across drivers
+ * see &dma_buf.resv.
+ */
struct dma_resv_list __rcu *fence;
};

@@ -98,6 +172,13 @@ static inline void dma_resv_reset_shared_max(struct dma_resv *obj) {}
* undefined order, a #ww_acquire_ctx is passed to unwind if a cycle
* is detected. See ww_mutex_lock() and ww_acquire_init(). A reservation
* object may be locked by itself by passing NULL as @ctx.
+ *
+ * When a die situation is indicated by returning -EDEADLK all locks held by
+ * @ctx must be unlocked and then dma_resv_lock_slow() called on @obj.
+ *
+ * Unlocked by calling dma_resv_unlock().
+ *
+ * See also dma_resv_lock_interruptible() for the interruptible variant.
*/
static inline int dma_resv_lock(struct dma_resv *obj,
struct ww_acquire_ctx *ctx)
@@ -119,6 +200,12 @@ static inline int dma_resv_lock(struct dma_resv *obj,
* undefined order, a #ww_acquire_ctx is passed to unwind if a cycle
* is detected. See ww_mutex_lock() and ww_acquire_init(). A reservation
* object may be locked by itself by passing NULL as @ctx.
+ *
+ * When a die situation is indicated by returning -EDEADLK all locks held by
+ * @ctx must be unlocked and then dma_resv_lock_slow_interruptible() called on
+ * @obj.
+ *
+ * Unlocked by calling dma_resv_unlock().
*/
static inline int dma_resv_lock_interruptible(struct dma_resv *obj,
struct ww_acquire_ctx *ctx)
@@ -134,6 +221,8 @@ static inline int dma_resv_lock_interruptible(struct dma_resv *obj,
* Acquires the reservation object after a die case. This function
* will sleep until the lock becomes available. See dma_resv_lock() as
* well.
+ *
+ * See also dma_resv_lock_slow_interruptible() for the interruptible variant.
*/
static inline void dma_resv_lock_slow(struct dma_resv *obj,
struct ww_acquire_ctx *ctx)
@@ -167,7 +256,7 @@ static inline int dma_resv_lock_slow_interruptible(struct dma_resv *obj,
* if they overlap with a writer.
*
* Also note that since no context is provided, no deadlock protection is
- * possible.
+ * possible, which is also not needed for a trylock.
*
* Returns true if the lock was acquired, false otherwise.
*/
@@ -193,6 +282,11 @@ static inline bool dma_resv_is_locked(struct dma_resv *obj)
*
* Returns the context used to lock a reservation object or NULL if no context
* was used or the object is not locked at all.
+ *
+ * WARNING: This interface is pretty horrible, but TTM needs it because it
+ * doesn't pass the struct ww_acquire_ctx around in some very long callchains.
+ * Everyone else just uses it to check whether they're holding a reservation or
+ * not.
*/
static inline struct ww_acquire_ctx *dma_resv_locking_ctx(struct dma_resv *obj)
{
--
2.32.0