On Tue, 2018-01-16 at 17:32 -0500, Mike Snitzer wrote:
> Therefore it seems to me that all queue_attr_{show,store} are racey vs
> blk_unregister_queue() removing the 'queue' kobject.
>
> And it was just that __elevator_change() was myopicly fixed to address
> the race whereas a more generic solution was/is needed. But short of
> that more generic fix your change will reintroduce the potential for
> hitting the issue that commit e9a823fb34a8b fixed.
>
> In that light, think it best to leave blk_unregister_queue()'s
> mutex_lock() above the QUEUE_FLAG_REGISTERED clearing _and_ update
> queue_attr_{show,store} to test for QUEUE_FLAG_REGISTERED while holding
> sysfs_lock.
>
> Then remove the unicorn test_bit for QUEUE_FLAG_REGISTERED from
> __elevator_change().
Thanks Mike for the feedback. However, I think a simpler approach exists than
what has been described above, namely by unregistering the sysfs attributes
earlier. How about the patch below?
[PATCH] block: Protect less code with sysfs_lock in blk_{un,}register_queue()
---
block/blk-sysfs.c | 39 ++++++++++++++++++++++++++-------------
block/elevator.c | 4 ----
2 files changed, 26 insertions(+), 17 deletions(-)
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 4a6a40ffd78e..ce32366c6db7 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;
@@ -934,28 +946,29 @@ void blk_unregister_queue(struct gendisk *disk)
if (!test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags))
return;
- /*
- * Protect against the 'queue' kobj being accessed
- * while/after it is removed.
- */
- mutex_lock(&q->sysfs_lock);
-
spin_lock_irq(q->queue_lock);
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.
+ */
+ mutex_lock(&q->sysfs_lock);
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);
}
diff --git a/block/elevator.c b/block/elevator.c
index e87e9b43aba0..4b7957b28a99 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -1080,10 +1080,6 @@ static int __elevator_change(struct request_queue *q, const char *name)
char elevator_name[ELV_NAME_MAX];
struct elevator_type *e;
- /* Make sure queue is not in the middle of being removed */
- if (!test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags))
- return -ENOENT;
-
/*
* Special case for mq, turn off scheduling
*/
--
2.15.1
