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

From: Paolo Valente <hidden>
Date: 2017-03-02 18:16:53

Il giorno 02 mar 2017, alle ore 17:13, Jens Axboe [off-list ref] ha =

scritto:

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On 03/02/2017 09:07 AM, Paolo Valente wrote:

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Il giorno 02 mar 2017, alle ore 16:13, Jens Axboe [off-list ref] ha =

scritto:

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On 03/02/2017 08:00 AM, Jens Axboe wrote:

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On 03/02/2017 03:28 AM, Paolo Valente wrote:

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Il giorno 15 feb 2017, alle ore 19:04, Jens Axboe [off-list ref] =

ha scritto:

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On 02/15/2017 10:58 AM, Jens Axboe wrote:

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On 02/15/2017 10:24 AM, Paolo Valente wrote:

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Il giorno 10 feb 2017, alle ore 19:32, Omar Sandoval =

[off-list ref] ha scritto:

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From: Omar Sandoval <redacted>
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None of the other blk-mq elevator hooks are called with this =

lock held.

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Additionally, it can lead to circular locking dependencies =

between

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queue_lock and the private scheduler lock.
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Hi Omar,
I'm sorry but it seems that a new potential deadlock has showed =

up.

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See lockdep splat below.
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I've tried to think about different solutions than turning back =

to

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deferring the body of exit_icq, but at no avail.

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Looks like a interaction between bfqd->lock and q->queue_lock. =

Since the

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core has no notion of you bfqd->lock, the naturally dependency =

here

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would be to nest bfqd->lock inside q->queue_lock. Is that =

possible for

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you?
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Looking at the code a bit, maybe it'd just be simpler to get rid =

of

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holding the queue lock for that spot. For the mq scheduler, we =

really

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don't want places where we invoke with that held already. Does =

the below

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work for you?
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Would need to remove one more lockdep assert. And only test this =

for

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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.
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Jens,
here is the reply I anticipated in my previous email: after =

rebasing

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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

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sort of equivalent commit (now) to the mainline branch.  Am I =

wrong?

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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?
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Let me take another stab at it today, I'll send you a version to =

test

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on top of my for-linus branch.

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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,

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it'd be great if you could verify that this works. Not for bfq-mq =

(we

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already know it does), but for CFQ.

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No luck, sorry :(
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It looks like to have just not to take q->queue_lock in cfq_exit_icq.

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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.

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It seems to be: no more crashes or lockdep complains after several =
tests, boots and shutdowns :)

Thanks,
Paolo

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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 |=3D ICQ_EXITED;
}
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-/* 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 =3D icq->ioc;

@@ -62,7 +62,6 @@ static void ioc_destroy_icq(struct io_cq *icq)

	struct elevator_type *et =3D q->elevator->type;
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	lockdep_assert_held(&ioc->lock);
-	lockdep_assert_held(q->queue_lock);
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	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);
}
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+static void __ioc_clear_queue(struct list_head *icq_list)
+{
+	unsigned long flags;
+
+	while (!list_empty(icq_list)) {
+		struct io_cq *icq =3D list_entry(icq_list->next,
+					       struct io_cq, q_node);
+		struct io_context *ioc =3D 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.

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+ * 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);
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-	while (!list_empty(&q->icq_list)) {
-		struct io_cq *icq =3D list_entry(q->icq_list.next,
-					       struct io_cq, q_node);
-		struct io_context *ioc =3D icq->ioc;
+	spin_lock_irq(q->queue_lock);
+	list_splice_init(&q->icq_list, &icq_list);
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-		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);
	}
}
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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)

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	blkcg_exit_queue(q);
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	if (q->elevator) {
-		spin_lock_irq(q->queue_lock);
		ioc_clear_queue(q);
-		spin_unlock_irq(q->queue_lock);
		elevator_exit(q->elevator);
	}
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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);
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-		spin_lock_irq(q->queue_lock);
		ioc_clear_queue(q);
-		spin_unlock_irq(q->queue_lock);
	}
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	/* allocate, init and register new elevator */
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Jens Axboe

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