2 files changed, 27 insertions, 7 deletions

diff --git a/block/blk-core.c b/block/blk-core.c
index 3508751c779a..034cbb2024f0 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1203,18 +1203,32 @@ static bool bio_attempt_front_merge(struct request_queue *q,
         return true;
 }
 
-/*
- * Attempts to merge with the plugged list in the current process. Returns
- * true if merge was successful, otherwise false.
+/**
+ * attempt_plug_merge - try to merge with %current's plugged list
+ * @q: request_queue new bio is being queued at
+ * @bio: new bio being queued
+ * @request_count: out parameter for number of traversed plugged requests
+ *
+ * Determine whether @bio being queued on @q can be merged with a request
+ * on %current's plugged list.  Returns %true if merge was successful,
+ * otherwise %false.
+ *
+ * This function is called without @q->queue_lock; however, elevator is
+ * accessed iff there already are requests on the plugged list which in
+ * turn guarantees validity of the elevator.
+ *
+ * Note that, on successful merge, elevator operation
+ * elevator_bio_merged_fn() will be called without queue lock.  Elevator
+ * must be ready for this.
  */
-static bool attempt_plug_merge(struct task_struct *tsk, struct request_queue *q,
-                               struct bio *bio, unsigned int *request_count)
+static bool attempt_plug_merge(struct request_queue *q, struct bio *bio,
+                               unsigned int *request_count)
 {
         struct blk_plug *plug;
         struct request *rq;
         bool ret = false;
 
-        plug = tsk->plug;
+        plug = current->plug;
         if (!plug)
                 goto out;
         *request_count = 0;
@@ -1282,7 +1296,7 @@ void blk_queue_bio(struct request_queue *q, struct bio *bio)
          * Check if we can merge with the plugged list before grabbing
          * any locks.
          */
-        if (attempt_plug_merge(current, q, bio, &request_count))
+        if (attempt_plug_merge(q, bio, &request_count))
                 return;
 
         spin_lock_irq(q->queue_lock);
diff --git a/include/linux/elevator.h b/include/linux/elevator.h
index d800d5142184..1d0f7a2ff73b 100644
--- a/include/linux/elevator.h
+++ b/include/linux/elevator.h
@@ -38,6 +38,12 @@ struct elevator_ops
         elevator_merged_fn *elevator_merged_fn;
         elevator_merge_req_fn *elevator_merge_req_fn;
         elevator_allow_merge_fn *elevator_allow_merge_fn;
+
+        /*
+         * Used for both plugged list and elevator merging and in the
+         * former case called without queue_lock.  Read comment on top of
+         * attempt_plug_merge() for details.
+         */
         elevator_bio_merged_fn *elevator_bio_merged_fn;
 
         elevator_dispatch_fn *elevator_dispatch_fn;