[PATCH 2/5] drm/ttm: Don't deadlock on recursive multi-bo reservations

[Thomas Hellstrom <thellstrom@vmware.com>]

Add an aid for the driver to detect deadlocks on multi-bo reservations
Update documentation.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
---
 drivers/gpu/drm/ttm/ttm_bo.c    |   15 ++++++++++++---
 include/drm/ttm/ttm_bo_driver.h |   25 ++++++++++++++++++++++---
 2 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
index 9ef893d..5d87508 100644
--- a/drivers/gpu/drm/ttm/ttm_bo.c
+++ b/drivers/gpu/drm/ttm/ttm_bo.c
@@ -223,9 +223,18 @@ int ttm_bo_reserve_locked(struct ttm_buffer_object *bo,
 		/**
 		 * Deadlock avoidance for multi-bo reserving.
 		 */
-		if (use_sequence && bo->seq_valid &&
-			(sequence - bo->val_seq < (1 << 31))) {
-			return -EAGAIN;
+		if (use_sequence && bo->seq_valid) {
+			/**
+			 * We've already reserved this one.
+			 */
+			if (unlikely(sequence == bo->val_seq))
+				return -EDEADLK;
+			/**
+			 * Already reserved by a thread that will not back
+			 * off for us. We need to back off.
+			 */
+			if (unlikely(sequence - bo->val_seq < (1 << 31)))
+				return -EAGAIN;
 		}
 
 		if (no_wait)
diff --git a/include/drm/ttm/ttm_bo_driver.h b/include/drm/ttm/ttm_bo_driver.h
index 95068e6..1e25a40 100644
--- a/include/drm/ttm/ttm_bo_driver.h
+++ b/include/drm/ttm/ttm_bo_driver.h
@@ -859,6 +859,9 @@ extern void ttm_bo_unmap_virtual(struct ttm_buffer_object *bo);
  * try again. (only if use_sequence == 1).
  * -ERESTARTSYS: A wait for the buffer to become unreserved was interrupted by
  * a signal. Release all buffer reservations and return to user-space.
+ * -EBUSY: The function needed to sleep, but @no_wait was true
+ * -EDEADLK: Bo already reserved using @sequence. This error code will only
+ * be returned if @use_sequence is set to true.
  */
 extern int ttm_bo_reserve(struct ttm_buffer_object *bo,
 			  bool interruptible,
@@ -868,11 +871,27 @@ extern int ttm_bo_reserve(struct ttm_buffer_object *bo,
 /**
  * ttm_bo_reserve_locked:
  *
- * Similar to ttm_bo_reserve, but must be called with the glob::lru_lock
- * spinlock held, and will not remove reserved buffers from the lru lists.
+ * @bo: A pointer to a struct ttm_buffer_object.
+ * @interruptible: Sleep interruptible if waiting.
+ * @no_wait: Don't sleep while trying to reserve, rather return -EBUSY.
+ * @use_sequence: If @bo is already reserved, Only sleep waiting for
+ * it to become unreserved if @sequence < (@bo)->sequence.
+ *
+ * Must be called with struct ttm_bo_global::lru_lock held,
+ * and will not remove reserved buffers from the lru lists.
  * The function may release the LRU spinlock if it needs to sleep.
+ * Otherwise identical to ttm_bo_reserve.
+ *
+ * Returns:
+ * -EAGAIN: The reservation may cause a deadlock.
+ * Release all buffer reservations, wait for @bo to become unreserved and
+ * try again. (only if use_sequence == 1).
+ * -ERESTARTSYS: A wait for the buffer to become unreserved was interrupted by
+ * a signal. Release all buffer reservations and return to user-space.
+ * -EBUSY: The function needed to sleep, but @no_wait was true
+ * -EDEADLK: Bo already reserved using @sequence. This error code will only
+ * be returned if @use_sequence is set to true.
  */
-
 extern int ttm_bo_reserve_locked(struct ttm_buffer_object *bo,
 				 bool interruptible,
 				 bool no_wait, bool use_sequence,
-- 
1.6.2.5