Merge v7.1.5linux-rolling-stable

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 10 insertions, 3 deletions

diff --git a/block/blk.h b/block/blk.h
index b998a7761faf..17e03656ba9a 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -689,6 +689,7 @@ static inline int req_ref_read(struct request *req)
 static inline u64 blk_time_get_ns(void)
 {
 	struct blk_plug *plug = current->plug;
+	u64 now;
 
 	if (!plug || !in_task())
 		return ktime_get_ns();
@@ -697,12 +698,18 @@ static inline u64 blk_time_get_ns(void)
 	 * 0 could very well be a valid time, but rather than flag "this is
 	 * a valid timestamp" separately, just accept that we'll do an extra
 	 * ktime_get_ns() if we just happen to get 0 as the current time.
+	 *
+	 * cur_ktime can be zeroed by pre-emption the moment PF_BLOCK_TS is set.
 	 */
-	if (!plug->cur_ktime) {
-		plug->cur_ktime = ktime_get_ns();
+	now = READ_ONCE(plug->cur_ktime);
+	if (!now) {
+		now = ktime_get_ns();
+		WRITE_ONCE(plug->cur_ktime, now);
+		/* Ensure PF_BLOCK_TS is set after cur_ktime. */
+		barrier();
 		current->flags |= PF_BLOCK_TS;
 	}
-	return plug->cur_ktime;
+	return now;
 }
 
 static inline ktime_t blk_time_get(void)