Re: [PATCH] blk-mq-sched: don't hold queue_lock when calling exit_icq
From: Jens Axboe <axboe@fb.com>
Date: 2017-02-15 18:04:46
Subsystem:
block layer, the rest · Maintainers:
Jens Axboe, Linus Torvalds
On 02/15/2017 10:58 AM, Jens Axboe wrote:
On 02/15/2017 10:24 AM, Paolo Valente wrote:quoted
quoted
Il giorno 10 feb 2017, alle ore 19:32, Omar Sandoval [off-list ref] ha scritto: From: Omar Sandoval <redacted> 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