drm/ttm: Fix up a theoretical deadlock

A process suspended waiting for a higher sequence or no sequence to unreserve,
a bo may be beaten to the reservation by a process with a lower sequence.
In that case the first process should give up trying to reserve and
return -EAGAIN. In order for that to happen, we must wake waiting processes
when we change sequence, so that they have a chance to detect the new
sequence.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
index 3ca77dc03915..148a322d8f5d 100644
--- a/drivers/gpu/drm/ttm/ttm_bo.c
+++ b/drivers/gpu/drm/ttm/ttm_bo.c
@@ -224,6 +224,9 @@ int ttm_bo_reserve_locked(struct ttm_buffer_object *bo,
         int ret;
 
         while (unlikely(atomic_cmpxchg(&bo->reserved, 0, 1) != 0)) {
+                /**
+                 * Deadlock avoidance for multi-bo reserving.
+                 */
                 if (use_sequence && bo->seq_valid &&
                         (sequence - bo->val_seq < (1 << 31))) {
                         return -EAGAIN;
@@ -241,6 +244,14 @@ int ttm_bo_reserve_locked(struct ttm_buffer_object *bo,
         }
 
         if (use_sequence) {
+                /**
+                 * Wake up waiters that may need to recheck for deadlock,
+                 * if we decreased the sequence number.
+                 */
+                if (unlikely((bo->val_seq - sequence < (1 << 31))
+                             || !bo->seq_valid))
+                        wake_up_all(&bo->event_queue);
+
                 bo->val_seq = sequence;
                 bo->seq_valid = true;
         } else {