staging: erofs: add a full barrier in erofs_workgroup_unfreeze

commit 948bbdb1818b7ad6e539dad4fbd2dd4650793ea9 upstream.

Just like other generic locks, insert a full barrier
in case of memory reorder.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/erofs/internal.h

@@ -199,6 +199,11 @@ static inline bool erofs_workgroup_try_to_freeze(struct erofs_workgroup *grp,
 static inline void erofs_workgroup_unfreeze(struct erofs_workgroup *grp,
 					    int orig_val)
 {
+	/*
+	 * other observers should notice all modifications
+	 * in the freezing period.
+	 */
+	smp_mb();
 	atomic_set(&grp->refcount, orig_val);
 	preempt_enable();
 }