Re: [PATCH] blk-mq-sched: don't hold queue_lock when calling exit_icq

[Jens Axboe <axboe@fb.com>]

On 02/15/2017 10:58 AM, Jens Axboe wrote:
> On 02/15/2017 10:24 AM, Paolo Valente wrote:
>>
>>> Il giorno 10 feb 2017, alle ore 19:32, Omar Sandoval <osandov@osandov.com> ha scritto:
>>>
>>> From: Omar Sandoval <osandov@fb.com>
>>>
>>> None of the other blk-mq elevator hooks are called with this lock held.
>>> Additionally, it can lead to circular locking dependencies between
>>> queue_lock and the private scheduler lock.
>>>
>>
>> Hi Omar,
>> I'm sorry but it seems that a new potential deadlock has showed up.
>> See lockdep splat below.
>>
>> I've tried to think about different solutions than turning back to
>> deferring the body of exit_icq, but at no avail.
> 
> Looks like a interaction between bfqd->lock and q->queue_lock. Since the
> core has no notion of you bfqd->lock, the naturally dependency here
> would be to nest bfqd->lock inside q->queue_lock. Is that possible for
> you?
> 
> Looking at the code a bit, maybe it'd just be simpler to get rid of
> holding the queue lock for that spot. For the mq scheduler, we really
> don't want places where we invoke with that held already. Does the below
> work for you?

Would need to remove one more lockdep assert. And only test this for
the mq parts, we'd need to spread a bit of love on the classic
scheduling icq exit path for this to work on that side.

diff --git a/block/blk-ioc.c b/block/blk-ioc.c
index b12f9c87b4c3..546ff8f81ede 100644
--- a/block/blk-ioc.c
+++ b/block/blk-ioc.c
@@ -54,7 +54,7 @@ static void ioc_exit_icq(struct io_cq *icq)
 	icq->flags |= ICQ_EXITED;
 }
 
-/* Release an icq.  Called with both ioc and q locked. */
+/* Release an icq.  Called with ioc locked. */
 static void ioc_destroy_icq(struct io_cq *icq)
 {
 	struct io_context *ioc = icq->ioc;
@@ -62,7 +62,6 @@ static void ioc_destroy_icq(struct io_cq *icq)
 	struct elevator_type *et = q->elevator->type;
 
 	lockdep_assert_held(&ioc->lock);
-	lockdep_assert_held(q->queue_lock);
 
 	radix_tree_delete(&ioc->icq_tree, icq->q->id);
 	hlist_del_init(&icq->ioc_node);
@@ -222,25 +221,34 @@ void exit_io_context(struct task_struct *task)
 	put_io_context_active(ioc);
 }
 
+static void __ioc_clear_queue(struct list_head *icq_list)
+{
+	while (!list_empty(icq_list)) {
+		struct io_cq *icq = list_entry(icq_list->next,
+					       struct io_cq, q_node);
+		struct io_context *ioc = icq->ioc;
+
+		spin_lock_irq(&ioc->lock);
+		ioc_destroy_icq(icq);
+		spin_unlock_irq(&ioc->lock);
+	}
+}
+
 /**
  * ioc_clear_queue - break any ioc association with the specified queue
  * @q: request_queue being cleared
  *
- * Walk @q->icq_list and exit all io_cq's.  Must be called with @q locked.
+ * Walk @q->icq_list and exit all io_cq's.
  */
 void ioc_clear_queue(struct request_queue *q)
 {
-	lockdep_assert_held(q->queue_lock);
+	LIST_HEAD(icq_list);
 
-	while (!list_empty(&q->icq_list)) {
-		struct io_cq *icq = list_entry(q->icq_list.next,
-					       struct io_cq, q_node);
-		struct io_context *ioc = icq->ioc;
+	spin_lock_irq(q->queue_lock);
+	list_splice_init(&q->icq_list, &icq_list);
+	spin_unlock_irq(q->queue_lock);
 
-		spin_lock(&ioc->lock);
-		ioc_destroy_icq(icq);
-		spin_unlock(&ioc->lock);
-	}
+	__ioc_clear_queue(&icq_list);
 }
 
 int create_task_io_context(struct task_struct *task, gfp_t gfp_flags, int node)
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 070d81bae1d5..1944aa1cb899 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -815,9 +815,7 @@ static void blk_release_queue(struct kobject *kobj)
 	blkcg_exit_queue(q);
 
 	if (q->elevator) {
-		spin_lock_irq(q->queue_lock);
 		ioc_clear_queue(q);
-		spin_unlock_irq(q->queue_lock);
 		elevator_exit(q->elevator);
 	}
 
diff --git a/block/elevator.c b/block/elevator.c
index a25bdd90b270..aaa1e9836512 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -985,9 +985,7 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
 		if (old_registered)
 			elv_unregister_queue(q);
 
-		spin_lock_irq(q->queue_lock);
 		ioc_clear_queue(q);
-		spin_unlock_irq(q->queue_lock);
 	}
 
 	/* allocate, init and register new elevator */

-- 
Jens Axboe