> Il giorno 02 mar 2017, alle ore 16:13, Jens Axboe <axboe@xxxxxx> 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 <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.
>>>>
>>>
>>>
>>> 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.
[ 9.329501] =============================================
[ 9.336132] [ INFO: possible recursive locking detected ]
[ 9.343801] 4.10.0-bfq-mq+ #56 Not tainted
[ 9.353359] ---------------------------------------------
[ 9.354101] ata_id/160 is trying to acquire lock:
[ 9.354750] (&(&q->__queue_lock)->rlock){..-...}, at: [<ffffffffaf48501a>] cfq_exit_icq+0x2a/0x80
[ 9.355986]
[ 9.355986] but task is already holding lock:
[ 9.364221] (&(&q->__queue_lock)->rlock){..-...}, at: [<ffffffffaf45a686>] put_io_context_active+0x86/0xe0
[ 9.382980]
[ 9.382980] other info that might help us debug this:
[ 9.386460] Possible unsafe locking scenario:
[ 9.386460]
[ 9.397070] CPU0
[ 9.397468] ----
[ 9.397811] lock(&(&q->__queue_lock)->rlock);
[ 9.398440] lock(&(&q->__queue_lock)->rlock);
[ 9.406265]
[ 9.406265] *** DEADLOCK ***
[ 9.406265]
[ 9.409589] May be due to missing lock nesting notation
[ 9.409589]
[ 9.413040] 2 locks held by ata_id/160:
[ 9.413576] #0: (&(&ioc->lock)->rlock/1){......}, at: [<ffffffffaf45a638>] put_io_context_active+0x38/0xe0
[ 9.418069] #1: (&(&q->__queue_lock)->rlock){..-...}, at: [<ffffffffaf45a686>] put_io_context_active+0x86/0xe0
[ 9.441118]
[ 9.441118] stack backtrace:
[ 9.445113] CPU: 0 PID: 160 Comm: ata_id Not tainted 4.10.0-bfq-mq+ #56
[ 9.446210] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[ 9.457223] Call Trace:
[ 9.457636] dump_stack+0x85/0xc2
[ 9.458183] __lock_acquire+0x1834/0x1890
[ 9.458839] ? __lock_acquire+0x4af/0x1890
[ 9.464206] lock_acquire+0x11b/0x220
[ 9.471703] ? cfq_exit_icq+0x2a/0x80
[ 9.472225] _raw_spin_lock+0x3d/0x80
[ 9.472784] ? cfq_exit_icq+0x2a/0x80
[ 9.476336] cfq_exit_icq+0x2a/0x80
[ 9.486750] ioc_exit_icq+0x38/0x50
[ 9.487245] put_io_context_active+0x92/0xe0
[ 9.488049] exit_io_context+0x48/0x50
[ 9.488423] do_exit+0x7a8/0xc80
[ 9.488797] do_group_exit+0x50/0xd0
[ 9.496655] SyS_exit_group+0x14/0x20
[ 9.497170] entry_SYSCALL_64_fastpath+0x23/0xc6
[ 9.497808] RIP: 0033:0x7f224d1c1b98
[ 9.498302] RSP: 002b:00007ffd342666f8 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
[ 9.499360] RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f224d1c1b98
[ 9.511706] RDX: 0000000000000000 RSI: 000000000000003c RDI: 0000000000000000
[ 9.516390] RBP: 00007f224db02690 R08: 00000000000000e7 R09: ffffffffffffff90
[ 9.523044] R10: 00007f224d6d7250 R11: 0000000000000246 R12: 0000000000000016
[ 9.524015] R13: 0000000000000001 R14: 000055938a1c3010 R15: 00007ffd34266170
Thanks,
Paolo
> Run with lockdep enabled and see if
> it spits out anything. We'll hit this exit path very quickly, so the
> test doesn't need to be anything exhaustive.
>
>
> 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 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/cfq-iosched.c b/block/cfq-iosched.c
> index 137944777859..4a71c79de037 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -3658,6 +3658,8 @@ static void cfq_exit_icq(struct io_cq *icq)
> struct cfq_io_cq *cic = icq_to_cic(icq);
> struct cfq_data *cfqd = cic_to_cfqd(cic);
>
> + spin_lock(cfqd->queue->queue_lock);
> +
> if (cic_to_cfqq(cic, false)) {
> cfq_exit_cfqq(cfqd, cic_to_cfqq(cic, false));
> cic_set_cfqq(cic, NULL, false);
> @@ -3667,6 +3669,8 @@ static void cfq_exit_icq(struct io_cq *icq)
> cfq_exit_cfqq(cfqd, cic_to_cfqq(cic, true));
> cic_set_cfqq(cic, NULL, true);
> }
> +
> + spin_unlock(cfqd->queue->queue_lock);
> }
>
> static void cfq_init_prio_data(struct cfq_queue *cfqq, struct cfq_io_cq *cic)
> 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 */
>
> --
> Jens Axboe
