drm/msm: Protect obj->active_count under obj lock

Previously we only held obj lock in the _active_get() path, and relied
on atomic_dec_return() to not be racy in the _active_put() path where
obj lock was not held.

But this is a false sense of security.  Unlike obj lifetime refcnt,
where you do not expect to *increase* the refcnt after the last put
(which would mean that something has gone horribly wrong with the
object liveness reference counting), the active_count can increase
again from zero.  Racing _active_put()s and _active_get()s could leave
the obj on the wrong mm list.

But in the retire path, immediately after the _active_put(), the
_unpin_iova() would acquire obj lock.  So just move the locking earlier
and rely on that to protect obj->active_count.

Fixes: c5c1643cef7a ("drm/msm: Drop struct_mutex from the retire path")
Signed-off-by: Rob Clark <robdclark@chromium.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/drivers/gpu/drm/msm/msm_gem.h b/drivers/gpu/drm/msm/msm_gem.h
index d79e7019cc88..3355a48a023b 100644
--- a/drivers/gpu/drm/msm/msm_gem.h
+++ b/drivers/gpu/drm/msm/msm_gem.h
@@ -87,7 +87,7 @@ struct msm_gem_object {
 
 	char name[32]; /* Identifier to print for the debugfs files */
 
-	atomic_t active_count;
+	int active_count;
 };
 #define to_msm_bo(x) container_of(x, struct msm_gem_object, base)
 
@@ -185,7 +185,8 @@ msm_gem_is_locked(struct drm_gem_object *obj)
 
 static inline bool is_active(struct msm_gem_object *msm_obj)
 {
-	return atomic_read(&msm_obj->active_count);
+	WARN_ON(!msm_gem_is_locked(&msm_obj->base));
+	return msm_obj->active_count;
 }
 
 static inline bool is_purgeable(struct msm_gem_object *msm_obj)