[PATCH v2 0/3] Avoid that blk_{un,}register_queue() trigger lock inversion

[PATCH v2 0/3] Avoid that blk_{un,}register_queue() trigger lock inversion

[Bart Van Assche]

Hello Jens,

The three patches in this series are what I came up with after having
analyzed a lockdep complaint triggered by blk_unregister_queue(). Please
consider these patches for kernel v4.16.

Thanks,

Bart.

Changes compared to v1:
- Restored the code that protects against races between changing the scheduler
  from sysfs and unregistering a block layer queue.

Bart Van Assche (3):
  block: Unexport elv_register_queue() and elv_unregister_queue()
  block: Document scheduler modification locking requirements
  block: Protect less code with sysfs_lock in blk_{un,}register_queue()

 block/blk-sysfs.c        | 37 ++++++++++++++++++++++++++++---------
 block/blk.h              |  3 +++
 block/elevator.c         | 10 ++++++++--
 include/linux/elevator.h |  2 --
 4 files changed, 39 insertions(+), 13 deletions(-)

-- 
2.15.1

[PATCH v2 1/3] block: Unexport elv_register_queue() and elv_unregister_queue()

[Bart Van Assche]

These two functions are only called from inside the block layer so
unexport them.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
---
 block/blk.h              | 3 +++
 block/elevator.c         | 2 --
 include/linux/elevator.h | 2 --
 3 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/block/blk.h b/block/blk.h
index 662005e46560..46db5dc83dcb 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -162,6 +162,9 @@ static inline void elv_deactivate_rq(struct request_queue *q, struct request *rq
 		e->type->ops.sq.elevator_deactivate_req_fn(q, rq);
 }
 
+int elv_register_queue(struct request_queue *q);
+void elv_unregister_queue(struct request_queue *q);
+
 struct hd_struct *__disk_get_part(struct gendisk *disk, int partno);
 
 #ifdef CONFIG_FAIL_IO_TIMEOUT
diff --git a/block/elevator.c b/block/elevator.c
index 138faeb08a7c..4f00b53cd5fd 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -886,7 +886,6 @@ int elv_register_queue(struct request_queue *q)
 	}
 	return error;
 }
-EXPORT_SYMBOL(elv_register_queue);
 
 void elv_unregister_queue(struct request_queue *q)
 {
@@ -900,7 +899,6 @@ void elv_unregister_queue(struct request_queue *q)
 		wbt_enable_default(q);
 	}
 }
-EXPORT_SYMBOL(elv_unregister_queue);
 
 int elv_register(struct elevator_type *e)
 {
diff --git a/include/linux/elevator.h b/include/linux/elevator.h
index 3d794b3dc532..6d9e230dffd2 100644
--- a/include/linux/elevator.h
+++ b/include/linux/elevator.h
@@ -198,8 +198,6 @@ extern bool elv_attempt_insert_merge(struct request_queue *, struct request *);
 extern void elv_requeue_request(struct request_queue *, struct request *);
 extern struct request *elv_former_request(struct request_queue *, struct request *);
 extern struct request *elv_latter_request(struct request_queue *, struct request *);
-extern int elv_register_queue(struct request_queue *q);
-extern void elv_unregister_queue(struct request_queue *q);
 extern int elv_may_queue(struct request_queue *, unsigned int);
 extern void elv_completed_request(struct request_queue *, struct request *);
 extern int elv_set_request(struct request_queue *q, struct request *rq,
-- 
2.15.1

[PATCH v2 2/3] block: Document scheduler modification locking requirements

[Bart Van Assche]

This patch does not change any functionality.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
---
 block/elevator.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/block/elevator.c b/block/elevator.c
index 4f00b53cd5fd..e87e9b43aba0 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -869,6 +869,8 @@ int elv_register_queue(struct request_queue *q)
 	struct elevator_queue *e = q->elevator;
 	int error;
 
+	lockdep_assert_held(&q->sysfs_lock);
+
 	error = kobject_add(&e->kobj, &q->kobj, "%s", "iosched");
 	if (!error) {
 		struct elv_fs_entry *attr = e->type->elevator_attrs;
@@ -889,6 +891,8 @@ int elv_register_queue(struct request_queue *q)
 
 void elv_unregister_queue(struct request_queue *q)
 {
+	lockdep_assert_held(&q->sysfs_lock);
+
 	if (q) {
 		struct elevator_queue *e = q->elevator;
 
@@ -965,6 +969,8 @@ static int elevator_switch_mq(struct request_queue *q,
 {
 	int ret;
 
+	lockdep_assert_held(&q->sysfs_lock);
+
 	blk_mq_freeze_queue(q);
 	blk_mq_quiesce_queue(q);
 
@@ -1010,6 +1016,8 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
 	bool old_registered = false;
 	int err;
 
+	lockdep_assert_held(&q->sysfs_lock);
+
 	if (q->mq_ops)
 		return elevator_switch_mq(q, new_e);
 
-- 
2.15.1

[PATCH v2 3/3] block: Protect less code with sysfs_lock in blk_{un,}register_queue()

[Bart Van Assche]

The __blk_mq_register_dev(), blk_mq_unregister_dev(),
elv_register_queue() and elv_unregister_queue() calls need to be
protected with sysfs_lock but other code in these functions not.
Hence protect only this code with sysfs_lock. This patch fixes a
locking inversion issue in blk_unregister_queue() and also in an
error path of blk_register_queue(): it is not allowed to hold
sysfs_lock around the kobject_del(&q->kobj) call.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
---
 block/blk-sysfs.c | 37 ++++++++++++++++++++++++++++---------
 1 file changed, 28 insertions(+), 9 deletions(-)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 4a6a40ffd78e..cbea895a5547 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -853,6 +853,10 @@ struct kobj_type blk_queue_ktype = {
 	.release	= blk_release_queue,
 };
 
+/**
+ * blk_register_queue - register a block layer queue with sysfs
+ * @disk: Disk of which the request queue should be registered with sysfs.
+ */
 int blk_register_queue(struct gendisk *disk)
 {
 	int ret;
@@ -909,11 +913,12 @@ int blk_register_queue(struct gendisk *disk)
 	if (q->request_fn || (q->mq_ops && q->elevator)) {
 		ret = elv_register_queue(q);
 		if (ret) {
+			mutex_unlock(&q->sysfs_lock);
 			kobject_uevent(&q->kobj, KOBJ_REMOVE);
 			kobject_del(&q->kobj);
 			blk_trace_remove_sysfs(dev);
 			kobject_put(&dev->kobj);
-			goto unlock;
+			return ret;
 		}
 	}
 	ret = 0;
@@ -923,6 +928,13 @@ int blk_register_queue(struct gendisk *disk)
 }
 EXPORT_SYMBOL_GPL(blk_register_queue);
 
+/**
+ * blk_unregister_queue - counterpart of blk_register_queue()
+ * @disk: Disk of which the request queue should be unregistered from sysfs.
+ *
+ * Note: the caller is responsible for guaranteeing that this function is called
+ * after blk_register_queue() has finished.
+ */
 void blk_unregister_queue(struct gendisk *disk)
 {
 	struct request_queue *q = disk->queue;
@@ -935,8 +947,9 @@ void blk_unregister_queue(struct gendisk *disk)
 		return;
 
 	/*
-	 * Protect against the 'queue' kobj being accessed
-	 * while/after it is removed.
+	 * Since sysfs_remove_dir() prevents adding new directory entries
+	 * before removal of existing entries starts, protect against
+	 * concurrent elv_iosched_store() calls.
 	 */
 	mutex_lock(&q->sysfs_lock);
 
@@ -944,18 +957,24 @@ void blk_unregister_queue(struct gendisk *disk)
 	queue_flag_clear(QUEUE_FLAG_REGISTERED, q);
 	spin_unlock_irq(q->queue_lock);
 
-	wbt_exit(q);
-
+	/*
+	 * Remove the sysfs attributes before unregistering the queue data
+	 * structures that can be modified through sysfs.
+	 */
 	if (q->mq_ops)
 		blk_mq_unregister_dev(disk_to_dev(disk), q);
-
-	if (q->request_fn || (q->mq_ops && q->elevator))
-		elv_unregister_queue(q);
+	mutex_unlock(&q->sysfs_lock);
 
 	kobject_uevent(&q->kobj, KOBJ_REMOVE);
 	kobject_del(&q->kobj);
 	blk_trace_remove_sysfs(disk_to_dev(disk));
-	kobject_put(&disk_to_dev(disk)->kobj);
 
+	wbt_exit(q);
+
+	mutex_lock(&q->sysfs_lock);
+	if (q->request_fn || (q->mq_ops && q->elevator))
+		elv_unregister_queue(q);
 	mutex_unlock(&q->sysfs_lock);
+
+	kobject_put(&disk_to_dev(disk)->kobj);
 }
-- 
2.15.1

Re: [PATCH v2 1/3] block: Unexport elv_register_queue() and elv_unregister_queue()

[Christoph Hellwig]

Looks fine,

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH v2 2/3] block: Document scheduler modification locking requirements

[Christoph Hellwig]

Looks fine,

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH v2 3/3] block: Protect less code with sysfs_lock in blk_{un,}register_queue()

[Christoph Hellwig]

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH v2 0/3] Avoid that blk_{un,}register_queue() trigger lock inversion

[Jens Axboe]

On 1/17/18 12:48 PM, Bart Van Assche wrote:
> Hello Jens,
> 
> The three patches in this series are what I came up with after having
> analyzed a lockdep complaint triggered by blk_unregister_queue(). Please
> consider these patches for kernel v4.16.

Thanks Bart, applied.

-- 
Jens Axboe