[PATCH V2 7/8] block: introduce preempt version of blk_[freeze|unfreeze]_queue

Ming Lei
2017-09-01T18:49:57Z

The two APIs are required to allow request allocation for
RQF_PREEMPT when queue is frozen.

The following two points have to be guaranteed for one queue:

1) preempt freezing can be started only after all pending
normal & preempt freezing are completed

2) normal freezing can't be started if there is pending
preempt freezing.

Because for normal freezing, once blk_mq_freeze_queue_wait()
is returned, we have to make sure no I/Os are pending.

rwsem should have been perfect for this kind of sync,
but lockdep will complain in case of nested normal freeze.

So spin_lock with freezing status is used for the sync.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
block/blk-core.c | 2 ++
block/blk-mq.c | 72 +++++++++++++++++++++++++++++++++++++++++++++++---
include/linux/blk-mq.h | 2 ++
include/linux/blkdev.h | 3 +++
4 files changed, 76 insertions(+), 3 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index c199910d4fe1..bbcea07f17da 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -899,6 +899,8 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
if (blkcg_init_queue(q))
goto fail_ref;

+ spin_lock_init(&q->freeze_lock);
+
return q;

fail_ref:
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 695d2eeaf41a..bf8c057aa50f 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -118,16 +118,48 @@ void blk_mq_in_flight(struct request_queue *q, struct hd_struct *part,
blk_mq_queue_tag_busy_iter(q, blk_mq_check_inflight, &mi);
}

-void blk_freeze_queue_start(struct request_queue *q)
+static void __blk_freeze_queue_start(struct request_queue *q, bool preempt)
{
int freeze_depth;

+ /*
+ * Wait for completion of another kind of freezing.
+ *
+ * We have to sync between normal freeze and preempt
+ * freeze. preempt freeze can only be started iff all
+ * pending normal & preempt freezing are completed,
+ * meantime normal freeze can be started only if there
+ * isn't pending preempt freezing.
+ *
+ * rwsem should have been perfect for this kind of sync,
+ * but lockdep will complain in case of nested normal freeze.
+ *
+ * So we have to use lock to do that manually.
+ */
+ spin_lock(&q->freeze_lock);
+ wait_event_cmd(q->mq_freeze_wq,
+ preempt ? !(q->normal_freezing + q->preempt_freezing) : !q->preempt_freezing,
+ spin_unlock(&q->freeze_lock),
+ spin_lock(&q->freeze_lock));
+
freeze_depth = atomic_inc_return(&q->mq_freeze_depth);
if (freeze_depth == 1) {
+ if (preempt)
+ q->preempt_freezing = 1;
+ else
+ q->normal_freezing = 1;
+ spin_unlock(&q->freeze_lock);
+
percpu_ref_kill(&q->q_usage_counter);
if (q->mq_ops)
blk_mq_run_hw_queues(q, false);
- }
+ } else
+ spin_unlock(&q->freeze_lock);
+}
+
+void blk_freeze_queue_start(struct request_queue *q)
+{
+ __blk_freeze_queue_start(q, false);
}
EXPORT_SYMBOL_GPL(blk_freeze_queue_start);

@@ -166,20 +198,54 @@ void blk_freeze_queue(struct request_queue *q)
}
EXPORT_SYMBOL_GPL(blk_freeze_queue);

-void blk_unfreeze_queue(struct request_queue *q)
+static void __blk_unfreeze_queue(struct request_queue *q, bool preempt)
{
int freeze_depth;

freeze_depth = atomic_dec_return(&q->mq_freeze_depth);
WARN_ON_ONCE(freeze_depth < 0);
if (!freeze_depth) {
+ spin_lock(&q->freeze_lock);
+ if (preempt)
+ q->preempt_freezing = 0;
+ else
+ q->normal_freezing = 0;
+ spin_unlock(&q->freeze_lock);
percpu_ref_reinit(&q->q_usage_counter);
wake_up_all(&q->mq_freeze_wq);
}
}
+
+void blk_unfreeze_queue(struct request_queue *q)
+{
+ __blk_unfreeze_queue(q, false);
+}
EXPORT_SYMBOL_GPL(blk_unfreeze_queue);

/*
+ * Once this function is returned, only allow to get request
+ * for preempt purpose, such as RQF_PREEMPT.
+ *
+ */
+void blk_freeze_queue_preempt(struct request_queue *q)
+{
+ __blk_freeze_queue_start(q, true);
+ blk_freeze_queue_wait(q);
+}
+EXPORT_SYMBOL_GPL(blk_freeze_queue_preempt);
+
+/*
+ * It is the caller's responsibility to make sure no new
+ * request can be allocated before calling this function.
+ */
+void blk_unfreeze_queue_preempt(struct request_queue *q)
+{
+ blk_freeze_queue_wait(q);
+ __blk_unfreeze_queue(q, true);
+}
+EXPORT_SYMBOL_GPL(blk_unfreeze_queue_preempt);
+
+/*
* FIXME: replace the scsi_internal_device_*block_nowait() calls in the
* mpt3sas driver such that this function can be removed.
*/
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 0ba5cb043172..596f433eb54c 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -259,6 +259,8 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
busy_tag_iter_fn *fn, void *priv);
void blk_freeze_queue(struct request_queue *q);
void blk_unfreeze_queue(struct request_queue *q);
+void blk_freeze_queue_preempt(struct request_queue *q);
+void blk_unfreeze_queue_preempt(struct request_queue *q);
void blk_freeze_queue_start(struct request_queue *q);
void blk_freeze_queue_wait(struct request_queue *q);
int blk_mq_freeze_queue_wait_timeout(struct request_queue *q,
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index a43422f5379a..2d62965e91eb 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -565,6 +565,9 @@ struct request_queue {

int bypass_depth;
atomic_t mq_freeze_depth;
+ spinlock_t freeze_lock;
+ unsigned normal_freezing:1;
+ unsigned preempt_freezing:1;

#if defined(CONFIG_BLK_DEV_BSG)
bsg_job_fn *bsg_job_fn;
--
2.9.5