blk-mq-sched: don't hold queue_lock when calling exit_icq

Commit Message

Jens Axboe March 2, 2017, 4:13 p.m. UTC

On 03/02/2017 09:07 AM, Paolo Valente wrote:
> 
>> Il giorno 02 mar 2017, alle ore 16:13, Jens Axboe <[e-mail]> ha scritto:
>>
>> On 03/02/2017 08:00 AM, Jens Axboe wrote:
>>> On 03/02/2017 03:28 AM, Paolo Valente wrote:
>>>>
>>>>> Il giorno 15 feb 2017, alle ore 19:04, Jens Axboe <[e-mail]> ha scritto:
>>>>>
>>>>> 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 <[e-mail]> ha scritto:
>>>>>>>>
>>>>>>>> From: Omar Sandoval <[e-mail]>
>>>>>>>>
>>>>>>>> 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.
>>>>>
>>>>
>>>>
>>>> Jens,
>>>> here is the reply I anticipated in my previous email: after rebasing
>>>> against master, I'm getting again the deadlock that this patch of
>>>> yours solved (together with some changes in bfq-mq).  I thought you added a
>>>> sort of equivalent commit (now) to the mainline branch.  Am I wrong?
>>>
>>> The patch I posted was never pulled to completion, it wasn't clear
>>> to me if it fixed your issue or not. Maybe I missed a reply on
>>> that?
>>>
>>> Let me take another stab at it today, I'll send you a version to test
>>> on top of my for-linus branch.
>>
>> I took at look at my last posting, and I think it was only missing a
>> lock grab in cfq, since we don't hold that for icq exit anymore.  Paolo,
>> it'd be great if you could verify that this works. Not for bfq-mq (we
>> already know it does), but for CFQ.
> 
> No luck, sorry :(
> 
> It looks like to have just not to take q->queue_lock in cfq_exit_icq.

I was worried about that. How about the below? We need to grab the lock
at some point for legacy scheduling, but the ordering should be correct.

Comments

Omar Sandoval March 2, 2017, 8:53 p.m. UTC | #1

On Thu, Mar 02, 2017 at 09:13:30AM -0700, Jens Axboe wrote:
> I was worried about that. How about the below? We need to grab the lock
> at some point for legacy scheduling, but the ordering should be correct.

Makes sense, now the locking is consistent with the other place we call
ioc_exit_icq(). One nit below, and you can add

Reviewed-by: Omar Sandoval <[e-mail]>

> diff --git a/block/blk-ioc.c b/block/blk-ioc.c
> index b12f9c87b4c3..6fd633b5d567 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. */

For ioc_exit_icq(), we have the more explicit comment

/*
 * Exit an icq. Called with both ioc and q locked for sq, only ioc locked for
 * mq.
 */

Could you document that here, too?

>  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);
> @@ -222,24 +221,40 @@ void exit_io_context(struct task_struct *task)
>  	put_io_context_active(ioc);
>  }

Jens Axboe March 2, 2017, 8:59 p.m. UTC | #2

On 03/02/2017 01:53 PM, Omar Sandoval wrote:
> On Thu, Mar 02, 2017 at 09:13:30AM -0700, Jens Axboe wrote:
>> I was worried about that. How about the below? We need to grab the lock
>> at some point for legacy scheduling, but the ordering should be correct.
> 
> Makes sense, now the locking is consistent with the other place we call
> ioc_exit_icq(). One nit below, and you can add
> 
> Reviewed-by: Omar Sandoval <[e-mail]>
> 
>> diff --git a/block/blk-ioc.c b/block/blk-ioc.c
>> index b12f9c87b4c3..6fd633b5d567 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. */
> 
> For ioc_exit_icq(), we have the more explicit comment
> 
> /*
>  * Exit an icq. Called with both ioc and q locked for sq, only ioc locked for
>  * mq.
>  */
> 
> Could you document that here, too?

Done, I've synced the two comments now. Thanks for the review!

Patch

diff --git a/block/blk-ioc.c b/block/blk-ioc.c
index b12f9c87b4c3..6fd633b5d567 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);
@@ -222,24 +221,40 @@  void exit_io_context(struct task_struct *task)
 	put_io_context_active(ioc);
 }
 
+static void __ioc_clear_queue(struct list_head *icq_list)
+{
+	unsigned long flags;
+
+	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_irqsave(&ioc->lock, flags);
+		ioc_destroy_icq(icq);
+		spin_unlock_irqrestore(&ioc->lock, flags);
+	}
+}
+
 /**
  * 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_lock(&ioc->lock);
-		ioc_destroy_icq(icq);
-		spin_unlock(&ioc->lock);
+	if (q->mq_ops) {
+		spin_unlock_irq(q->queue_lock);
+		__ioc_clear_queue(&icq_list);
+	} else {
+		__ioc_clear_queue(&icq_list);
+		spin_unlock_irq(q->queue_lock);
 	}
 }
 
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 002af836aa87..c44b321335f3 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 ac1c9f481a98..01139f549b5b 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -983,9 +983,7 @@  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 */