[PATCH v5 08/12] blk-mq: use QUEUE_FLAG_QUIESCED to quiesce queue

Ming Lei
2017-06-06T15:24:08Z

It is required that no dispatch can happen any more once
blk_mq_quiesce_queue() returns, and we don't have such requirement
on APIs of stopping queue.

But blk_mq_quiesce_queue() still may not block/drain dispatch in the
the case of BLK_MQ_S_START_ON_RUN, so use the new introduced flag of
QUEUE_FLAG_QUIESCED and evaluate it inside RCU read-side critical
sections for fixing this issue.

Also blk_mq_quiesce_queue() is implemented via stopping queue, which
limits its uses, and easy to cause race, because any queue restart in
other paths may break blk_mq_quiesce_queue(). With the introduced
flag of QUEUE_FLAG_QUIESCED, we don't need to depend on stopping queue
for quiescing any more.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
block/blk-mq-sched.c | 3 ++-
block/blk-mq.c | 11 ++++++++++-
include/linux/blk-mq.h | 4 ++++
include/linux/blkdev.h | 2 ++
4 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index c4e2afb9d12d..ec9885df324c 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -141,7 +141,8 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
bool did_work = false;
LIST_HEAD(rq_list);

- if (unlikely(blk_mq_hctx_stopped(hctx)))
+ /* RCU or SRCU read lock is needed before checking quiesced flag */
+ if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
return;

hctx->run++;
diff --git a/block/blk-mq.c b/block/blk-mq.c
index ed8e2edca1a7..7369810d3dc0 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -170,6 +170,10 @@ void blk_mq_quiesce_queue(struct request_queue *q)

__blk_mq_stop_hw_queues(q, true);

+ spin_lock_irq(q->queue_lock);
+ queue_flag_set(QUEUE_FLAG_QUIESCED, q);
+ spin_unlock_irq(q->queue_lock);
+
queue_for_each_hw_ctx(q, hctx, i) {
if (hctx->flags & BLK_MQ_F_BLOCKING)
synchronize_srcu(&hctx->queue_rq_srcu);
@@ -190,6 +194,10 @@ EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue);
*/
void blk_mq_unquiesce_queue(struct request_queue *q)
{
+ spin_lock_irq(q->queue_lock);
+ queue_flag_clear(QUEUE_FLAG_QUIESCED, q);
+ spin_unlock_irq(q->queue_lock);
+
blk_mq_start_stopped_hw_queues(q, true);
}
EXPORT_SYMBOL_GPL(blk_mq_unquiesce_queue);
@@ -1426,7 +1434,8 @@ static void __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
int ret;
bool run_queue = true;

- if (blk_mq_hctx_stopped(hctx)) {
+ /* RCU or SRCU read lock is needed before checking quiesced flag */
+ if (blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)) {
run_queue = false;
goto insert;
}
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index fc0fc45fc658..dc96ce3f5425 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -271,6 +271,10 @@ void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues);
*/
static inline void blk_mq_quiesce_queue_nowait(struct request_queue *q)
{
+ spin_lock_irq(q->queue_lock);
+ queue_flag_set(QUEUE_FLAG_QUIESCED, q);
+ spin_unlock_irq(q->queue_lock);
+
blk_mq_stop_hw_queues(q);
}

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 7178ad6805e3..7da799a88244 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -619,6 +619,7 @@ struct request_queue {
#define QUEUE_FLAG_POLL_STATS 28 /* collecting stats for hybrid polling */
#define QUEUE_FLAG_REGISTERED 29 /* queue has been registered to a disk */
#define QUEUE_FLAG_SCSI_PASSTHROUGH 30 /* queue supports SCSI commands */
+#define QUEUE_FLAG_QUIESCED 31 /* queue has been quiesced */

#define QUEUE_FLAG_DEFAULT ((1 << QUEUE_FLAG_IO_STAT) | \
(1 << QUEUE_FLAG_STACKABLE) | \
@@ -715,6 +716,7 @@ static inline void queue_flag_clear(unsigned int flag, struct request_queue *q)
#define blk_noretry_request(rq) \
((rq)->cmd_flags & (REQ_FAILFAST_DEV|REQ_FAILFAST_TRANSPORT| \
REQ_FAILFAST_DRIVER))
+#define blk_queue_quiesced(q) test_bit(QUEUE_FLAG_QUIESCED, &(q)->queue_flags)

static inline bool blk_account_rq(struct request *rq)
{
--
2.9.4