block: extend queue bypassing to cover blkcg policies

Extend queue bypassing such that dying queue is always bypassing and
blk-throttle is drained on bypass.  With blkcg policies updated to
test blk_queue_bypass() instead of blk_queue_dead(), this ensures that
no bio or request is held by or going through blkcg policies on a
bypassing queue.

This will be used to implement blkg cleanup on elevator switches and
policy changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -372,7 +372,6 @@ void blk_drain_queue(struct request_queue *q, bool drain_all)
 		if (q->elevator)
 			elv_drain_elevator(q);
 
-		if (drain_all)
 		blk_throtl_drain(q);
 
 		/*
@@ -415,8 +414,8 @@ void blk_drain_queue(struct request_queue *q, bool drain_all)
  *
  * In bypass mode, only the dispatch FIFO queue of @q is used.  This
  * function makes @q enter bypass mode and drains all requests which were
- * issued before.  On return, it's guaranteed that no request has ELVPRIV
- * set.
+ * throttled or issued before.  On return, it's guaranteed that no request
+ * is being throttled or has ELVPRIV set.
  */
 void blk_queue_bypass_start(struct request_queue *q)
 {
@@ -461,6 +460,11 @@ void blk_cleanup_queue(struct request_queue *q)
 	queue_flag_set_unlocked(QUEUE_FLAG_DEAD, q);
 
 	spin_lock_irq(lock);
+
+	/* dead queue is permanently in bypass mode till released */
+	q->bypass_depth++;
+	queue_flag_set(QUEUE_FLAG_BYPASS, q);
+
 	queue_flag_set(QUEUE_FLAG_NOMERGES, q);
 	queue_flag_set(QUEUE_FLAG_NOXMERGES, q);
 	queue_flag_set(QUEUE_FLAG_DEAD, q);

block/blk-throttle.c

@@ -310,7 +310,7 @@ static struct throtl_grp * throtl_get_tg(struct throtl_data *td)
 	struct request_queue *q = td->queue;
 
 	/* no throttling for dead queue */
-	if (unlikely(blk_queue_dead(q)))
+	if (unlikely(blk_queue_bypass(q)))
 		return NULL;
 
 	rcu_read_lock();
@@ -335,7 +335,7 @@ static struct throtl_grp * throtl_get_tg(struct throtl_data *td)
 	spin_lock_irq(q->queue_lock);
 
 	/* Make sure @q is still alive */
-	if (unlikely(blk_queue_dead(q))) {
+	if (unlikely(blk_queue_bypass(q))) {
 		kfree(tg);
 		return NULL;
 	}