From: Yu Kuai <yukuai3@xxxxxxxxxx>
request_queue->nr_requests can be changed by:
a) switch elevator by updating nr_hw_queues
b) switch elevator by elevator sysfs attribute
c) configue queue sysfs attribute nr_requests
Current lock order is:
1) update_nr_hwq_lock, case a,b
2) freeze_queue
3) elevator_lock, case a,b,c
And update nr_requests is seriablized by elevator_lock() already,
however, in the case c, we'll have to allocate new sched_tags if
nr_requests grow, and do this with elevator_lock held and queue
freezed has the risk of deadlock.
Hence use update_nr_hwq_lock instead, make it possible to allocate
memory if tags grow, meanwhile also prevent nr_requests to be changed
concurrently.
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
block/blk-sysfs.c | 25 ++++++++++++++++++++-----
1 file changed, 20 insertions(+), 5 deletions(-)
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index f99519f7a820..ff66d1b47169 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -68,13 +68,17 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count)
int ret, err;
unsigned int memflags;
struct request_queue *q = disk->queue;
+ struct blk_mq_tag_set *set = q->tag_set;
ret = queue_var_store(&nr, page, count);
if (ret < 0)
return ret;
- memflags = blk_mq_freeze_queue(q);
- mutex_lock(&q->elevator_lock);
+ /*
+ * Serialize updating nr_requests with concurrent queue_requests_store()
+ * and switching elevator.
+ */
+ down_write(&set->update_nr_hwq_lock);
if (nr == q->nr_requests)
goto unlock;
@@ -82,20 +86,31 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count)
if (nr < BLKDEV_MIN_RQ)
nr = BLKDEV_MIN_RQ;
- if (nr <= q->tag_set->reserved_tags ||
+ /*
+ * Switching elevator is protected by update_nr_hwq_lock:
+ * - read lock is held from elevator sysfs attribute;
+ * - write lock is held from updating nr_hw_queues;
+ * Hence it's safe to access q->elevator here with write lock held.
+ */
+ if (nr <= set->reserved_tags ||
(q->elevator && nr > MAX_SCHED_RQ) ||
- (!q->elevator && nr > q->tag_set->queue_depth)) {
+ (!q->elevator && nr > set->queue_depth)) {
ret = -EINVAL;
goto unlock;
}
+ memflags = blk_mq_freeze_queue(q);
+ mutex_lock(&q->elevator_lock);
+
err = blk_mq_update_nr_requests(disk->queue, nr);
if (err)
ret = err;
-unlock:
mutex_unlock(&q->elevator_lock);
blk_mq_unfreeze_queue(q, memflags);
+
+unlock:
+ up_write(&set->update_nr_hwq_lock);
return ret;
}
--
2.39.2
