[PATCH v2 1/4] block: more locking around delayed work

[Giuliano Procida <gprocida@google.com>]

commit 287922eb0b186e2a5bf54fdd04b734c25c90035c upstream.

block: defer timeouts to a workqueue

Timer context is not very useful for drivers to perform any meaningful abort
action from.  So instead of calling the driver from this useless context
defer it to a workqueue as soon as possible.

Note that while a delayed_work item would seem the right thing here I didn't
dare to use it due to the magic in blk_add_timer that pokes deep into timer
internals.  But maybe this encourages Tejun to add a sensible API for that to
the workqueue API and we'll all be fine in the end :)

Contains a major update from Keith Bush:

"This patch removes synchronizing the timeout work so that the timer can
 start a freeze on its own queue. The timer enters the queue, so timer
 context can only start a freeze, but not wait for frozen."

NOTE: Back-ported to 4.4.y.

The only parts of the upstream commit that have been kept are various
locking changes, none of which were mentioned in the original commit
message which therefore describes this change not at all.

Timeout callbacks continue to be run via a timer. Both blk_mq_rq_timer
and blk_rq_timed_out_timer will return without without doing any work
if they cannot acquire the queue (without waiting).

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Giuliano Procida <gprocida@google.com>
---
 block/blk-mq.c      | 4 ++++
 block/blk-timeout.c | 3 +++
 2 files changed, 7 insertions(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 8649dbf06ce4..11a23bf73fd9 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -628,6 +628,9 @@ static void blk_mq_rq_timer(unsigned long priv)
 	};
 	int i;
 
+	if (blk_queue_enter(q, GFP_NOWAIT))
+		return;
+
 	blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &data);
 
 	if (data.next_set) {
@@ -642,6 +645,7 @@ static void blk_mq_rq_timer(unsigned long priv)
 				blk_mq_tag_idle(hctx);
 		}
 	}
+	blk_queue_exit(q);
 }
 
 /*
diff --git a/block/blk-timeout.c b/block/blk-timeout.c
index aa40aa93381b..2bc03df554a6 100644
--- a/block/blk-timeout.c
+++ b/block/blk-timeout.c
@@ -134,6 +134,8 @@ void blk_rq_timed_out_timer(unsigned long data)
 	struct request *rq, *tmp;
 	int next_set = 0;
 
+	if (blk_queue_enter(q, GFP_NOWAIT))
+		return;
 	spin_lock_irqsave(q->queue_lock, flags);
 
 	list_for_each_entry_safe(rq, tmp, &q->timeout_list, timeout_list)
@@ -143,6 +145,7 @@ void blk_rq_timed_out_timer(unsigned long data)
 		mod_timer(&q->timeout, round_jiffies_up(next));
 
 	spin_unlock_irqrestore(q->queue_lock, flags);
+	blk_queue_exit(q);
 }
 
 /**
-- 
2.26.0.110.g2183baf09c-goog