drm/ttm: Don't deadlock on recursive multi-bo reservations

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

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jerome Glisse <j.glisse@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 12 insertions, 3 deletions

diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
index 9ef893d5da88..5d8750830dc3 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)