linux-kernel - [PATCH 3/6] block, blk-mq: draining can't be skipped even if bypass

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Date:	Wed, 18 Jun 2014 11:21:09 -0400
From:	Tejun Heo <tj@...nel.org>
To:	axboe@...nel.dk
Cc:	linux-kernel@...r.kernel.org, kmo@...erainc.com,
	nab@...ux-iscsi.org, Tejun Heo <tj@...nel.org>
Subject: [PATCH 3/6] block, blk-mq: draining can't be skipped even if bypass_depth was non-zero

Currently, both blk_queeu_bypass_start() and blk_mq_freeze_queue()
skip queue draining if bypass_depth was already above zero.  The
assumption is that the one which bumped the bypass_depth should have
performed draining already; however, there's nothing which prevents a
new instance of bypassing/freezing from starting before the previous
one finishes draining.  The current code may allow the later
bypassing/freezing instances to complete while there still are
in-flight requests which haven't finished draining.

Fix it by draining regardless of bypass_depth.  We still skip draining
from blk_queue_bypass_start() while the queue is initializing to avoid
introducing excessive delays during boot.  INIT_DONE setting is moved
above the initial blk_queue_bypass_end() so that bypassing attempts
can't slip inbetween.

Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Nicholas A. Bellinger <nab@...ux-iscsi.org>
---
 block/blk-core.c  | 11 +++++++----
 block/blk-mq.c    |  7 ++-----
 block/blk-sysfs.c |  2 +-
 3 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index f6f6b9a..2375130 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -438,14 +438,17 @@ static void __blk_drain_queue(struct request_queue *q, bool drain_all)
  */
 void blk_queue_bypass_start(struct request_queue *q)
 {
-	bool drain;
-
 	spin_lock_irq(q->queue_lock);
-	drain = !q->bypass_depth++;
+	q->bypass_depth++;
 	queue_flag_set(QUEUE_FLAG_BYPASS, q);
 	spin_unlock_irq(q->queue_lock);
 
-	if (drain) {
+	/*
+	 * Queues start drained.  Skip actual draining till init is
+	 * complete.  This avoids lenghty delays during queue init which
+	 * can happen many times during boot.
+	 */
+	if (blk_queue_init_done(q)) {
 		spin_lock_irq(q->queue_lock);
 		__blk_drain_queue(q, false);
 		spin_unlock_irq(q->queue_lock);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 1c09d57..c0122a4 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -131,15 +131,12 @@ static void __blk_mq_drain_queue(struct request_queue *q)
  */
 static void blk_mq_freeze_queue(struct request_queue *q)
 {
-	bool drain;
-
 	spin_lock_irq(q->queue_lock);
-	drain = !q->bypass_depth++;
+	q->bypass_depth++;
 	queue_flag_set(QUEUE_FLAG_BYPASS, q);
 	spin_unlock_irq(q->queue_lock);
 
-	if (drain)
-		__blk_mq_drain_queue(q);
+	__blk_mq_drain_queue(q);
 }
 
 void blk_mq_drain_queue(struct request_queue *q)
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 23321fb..4db5abf 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -554,8 +554,8 @@ int blk_register_queue(struct gendisk *disk)
 	 * Initialization must be complete by now.  Finish the initial
 	 * bypass from queue allocation.
 	 */
-	blk_queue_bypass_end(q);
 	queue_flag_set_unlocked(QUEUE_FLAG_INIT_DONE, q);
+	blk_queue_bypass_end(q);
 
 	ret = blk_trace_init_sysfs(dev);
 	if (ret)
-- 
1.9.3

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