> Il giorno 15 feb 2017, alle ore 19:04, Jens Axboe <axboe@xxxxxx> 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 <osandov@xxxxxxxxxxx> ha scritto:
>>>>
>>>> From: Omar Sandoval <osandov@xxxxxx>
>>>>
>>>> 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.
>
Sorry Jens, same splat. What confuses me is the second column
in the possible scenario:
[ 139.368477] CPU0 CPU1
[ 139.369129] ---- ----
[ 139.369774] lock(&(&ioc->lock)->rlock);
[ 139.370339] lock(&(&q->__queue_lock)->rlock);
[ 139.390579] lock(&(&ioc->lock)->rlock);
[ 139.391522] lock(&(&bfqd->lock)->rlock);
I could not find any code path, related to the reported call traces,
and taking first q->queue_lock and then ioc->lock.
Any suggestion on how to go on, and hopefully help with this problem is
welcome.
Thanks,
Paolo
> 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
