The following is a description of a hang in blk_mq_freeze_queue_wait(). The hang happens on attempting to freeze a queue while another task does queue unfreeze. The root cause is an incorrect sequence of percpu_ref_resurrect() and percpu_ref_kill() and as a result those two can be swapped: CPU#0 CPU#1 ---------------- ----------------- q1 = blk_mq_init_queue(shared_tags) q2 = blk_mq_init_queue(shared_tags) blk_mq_add_queue_tag_set(shared_tags) blk_mq_update_tag_set_depth(shared_tags) list_for_each_entry() blk_mq_freeze_queue(q1) > percpu_ref_kill() > blk_mq_freeze_queue_wait() blk_cleanup_queue(q1) blk_mq_freeze_queue(q1) > percpu_ref_kill() ^^^^^^ freeze_depth can't guarantee the order blk_mq_unfreeze_queue() > percpu_ref_resurrect() > blk_mq_freeze_queue_wait() ^^^^^^ Hang here!!!! This wrong sequence raises kernel warning: percpu_ref_kill_and_confirm called more than once on blk_queue_usage_counter_release! WARNING: CPU: 0 PID: 11854 at lib/percpu-refcount.c:336 percpu_ref_kill_and_confirm+0x99/0xb0 But the most unpleasant effect is a hang of a blk_mq_freeze_queue_wait(), which waits for a zero of a q_usage_counter, which never happens because percpu-ref was reinited (instead of being killed) and stays in PERCPU state forever. How to reproduce: - "insmod null_blk.ko shared_tags=1 nr_devices=0 queue_mode=2" - cpu0: python Script.py 0; taskset the corresponding process running on cpu0 - cpu1: python Script.py 1; taskset the corresponding process running on cpu1 Script.py: ------ #!/usr/bin/python3 import os import sys while True: on = "echo 1 > /sys/kernel/config/nullb/%s/power" % sys.argv[1] off = "echo 0 > /sys/kernel/config/nullb/%s/power" % sys.argv[1] os.system(on) os.system(off) ------ This bug was first reported and fixed by Roman, previous discussion: [1] Message id: 1443287365-4244-7-git-send-email-akinobu.mita@gmail.com [2] Message id: 1443563240-29306-6-git-send-email-tj@kernel.org [3] https://patchwork.kernel.org/patch/9268199/ Signed-off-by: Bob Liu <bob.liu@oracle.com> --- v4: - Update commit log v3: - rebase to v5.1 v2: - forgotten hunk from local repo - minor tweaks in the commit message --- block/blk-core.c | 3 ++- block/blk-mq.c | 19 ++++++++++--------- include/linux/blkdev.h | 7 ++++++- 3 files changed, 18 insertions(+), 11 deletions(-) diff --git a/block/blk-core.c b/block/blk-core.c index a55389b..fb97497 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -433,7 +433,7 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags) smp_rmb(); wait_event(q->mq_freeze_wq, - (atomic_read(&q->mq_freeze_depth) == 0 && + (!q->mq_freeze_depth && (pm || (blk_pm_request_resume(q), !blk_queue_pm_only(q)))) || blk_queue_dying(q)); @@ -523,6 +523,7 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id) spin_lock_init(&q->queue_lock); init_waitqueue_head(&q->mq_freeze_wq); + mutex_init(&q->mq_freeze_lock); /* * Init percpu_ref in atomic mode so that it's faster to shutdown. diff --git a/block/blk-mq.c b/block/blk-mq.c index a935483..373af60 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -143,13 +143,14 @@ void blk_mq_in_flight_rw(struct request_queue *q, struct hd_struct *part, void blk_freeze_queue_start(struct request_queue *q) { - int freeze_depth; - - freeze_depth = atomic_inc_return(&q->mq_freeze_depth); - if (freeze_depth == 1) { + mutex_lock(&q->mq_freeze_lock); + if (++q->mq_freeze_depth == 1) { percpu_ref_kill(&q->q_usage_counter); + mutex_unlock(&q->mq_freeze_lock); if (queue_is_mq(q)) blk_mq_run_hw_queues(q, false); + } else { + mutex_unlock(&q->mq_freeze_lock); } } EXPORT_SYMBOL_GPL(blk_freeze_queue_start); @@ -198,14 +199,14 @@ EXPORT_SYMBOL_GPL(blk_mq_freeze_queue); void blk_mq_unfreeze_queue(struct request_queue *q) { - int freeze_depth; - - freeze_depth = atomic_dec_return(&q->mq_freeze_depth); - WARN_ON_ONCE(freeze_depth < 0); - if (!freeze_depth) { + mutex_lock(&q->mq_freeze_lock); + q->mq_freeze_depth--; + WARN_ON_ONCE(q->mq_freeze_depth < 0); + if (!q->mq_freeze_depth) { percpu_ref_resurrect(&q->q_usage_counter); wake_up_all(&q->mq_freeze_wq); } + mutex_unlock(&q->mq_freeze_lock); } EXPORT_SYMBOL_GPL(blk_mq_unfreeze_queue); diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h index 5c58a3b..64f7683 100644 --- a/include/linux/blkdev.h +++ b/include/linux/blkdev.h @@ -535,7 +535,7 @@ struct request_queue { struct mutex sysfs_lock; - atomic_t mq_freeze_depth; + int mq_freeze_depth; #if defined(CONFIG_BLK_DEV_BSG) struct bsg_class_device bsg_dev; @@ -547,6 +547,11 @@ struct request_queue { #endif struct rcu_head rcu_head; wait_queue_head_t mq_freeze_wq; + /* + * Protect concurrent access to q_usage_counter by + * percpu_ref_kill() and percpu_ref_reinit(). + */ + struct mutex mq_freeze_lock; struct percpu_ref q_usage_counter; struct list_head all_q_node; -- 2.9.5
On Thu, Apr 25, 2019 at 9:21 PM Bob Liu <bob.liu@oracle.com> wrote:
>
> The following is a description of a hang in blk_mq_freeze_queue_wait().
> The hang happens on attempting to freeze a queue while another task does
> queue unfreeze.
>
> The root cause is an incorrect sequence of percpu_ref_resurrect() and
> percpu_ref_kill() and as a result those two can be swapped:
>
> CPU#0 CPU#1
> ---------------- -----------------
> q1 = blk_mq_init_queue(shared_tags)
>
> q2 = blk_mq_init_queue(shared_tags)
> blk_mq_add_queue_tag_set(shared_tags)
> blk_mq_update_tag_set_depth(shared_tags)
> list_for_each_entry()
> blk_mq_freeze_queue(q1)
> > percpu_ref_kill()
> > blk_mq_freeze_queue_wait()
>
> blk_cleanup_queue(q1)
> blk_mq_freeze_queue(q1)
> > percpu_ref_kill()
> ^^^^^^ freeze_depth can't guarantee the order
>
> blk_mq_unfreeze_queue()
> > percpu_ref_resurrect()
>
> > blk_mq_freeze_queue_wait()
> ^^^^^^ Hang here!!!!
>
> This wrong sequence raises kernel warning:
> percpu_ref_kill_and_confirm called more than once on blk_queue_usage_counter_release!
> WARNING: CPU: 0 PID: 11854 at lib/percpu-refcount.c:336 percpu_ref_kill_and_confirm+0x99/0xb0
>
> But the most unpleasant effect is a hang of a blk_mq_freeze_queue_wait(),
> which waits for a zero of a q_usage_counter, which never happens
> because percpu-ref was reinited (instead of being killed) and stays in
> PERCPU state forever.
>
> How to reproduce:
> - "insmod null_blk.ko shared_tags=1 nr_devices=0 queue_mode=2"
> - cpu0: python Script.py 0; taskset the corresponding process running on cpu0
> - cpu1: python Script.py 1; taskset the corresponding process running on cpu1
>
> Script.py:
> ------
> #!/usr/bin/python3
>
> import os
> import sys
>
> while True:
> on = "echo 1 > /sys/kernel/config/nullb/%s/power" % sys.argv[1]
> off = "echo 0 > /sys/kernel/config/nullb/%s/power" % sys.argv[1]
> os.system(on)
> os.system(off)
> ------
>
> This bug was first reported and fixed by Roman, previous discussion:
> [1] Message id: 1443287365-4244-7-git-send-email-akinobu.mita@gmail.com
> [2] Message id: 1443563240-29306-6-git-send-email-tj@kernel.org
> [3] https://patchwork.kernel.org/patch/9268199/
>
> Signed-off-by: Bob Liu <bob.liu@oracle.com>
> ---
> v4:
> - Update commit log
> v3:
> - rebase to v5.1
> v2:
> - forgotten hunk from local repo
> - minor tweaks in the commit message
> ---
> block/blk-core.c | 3 ++-
> block/blk-mq.c | 19 ++++++++++---------
> include/linux/blkdev.h | 7 ++++++-
> 3 files changed, 18 insertions(+), 11 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index a55389b..fb97497 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -433,7 +433,7 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
> smp_rmb();
>
> wait_event(q->mq_freeze_wq,
> - (atomic_read(&q->mq_freeze_depth) == 0 &&
> + (!q->mq_freeze_depth &&
> (pm || (blk_pm_request_resume(q),
> !blk_queue_pm_only(q)))) ||
> blk_queue_dying(q));
> @@ -523,6 +523,7 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
> spin_lock_init(&q->queue_lock);
>
> init_waitqueue_head(&q->mq_freeze_wq);
> + mutex_init(&q->mq_freeze_lock);
>
> /*
> * Init percpu_ref in atomic mode so that it's faster to shutdown.
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index a935483..373af60 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -143,13 +143,14 @@ void blk_mq_in_flight_rw(struct request_queue *q, struct hd_struct *part,
>
> void blk_freeze_queue_start(struct request_queue *q)
> {
> - int freeze_depth;
> -
> - freeze_depth = atomic_inc_return(&q->mq_freeze_depth);
> - if (freeze_depth == 1) {
> + mutex_lock(&q->mq_freeze_lock);
> + if (++q->mq_freeze_depth == 1) {
> percpu_ref_kill(&q->q_usage_counter);
> + mutex_unlock(&q->mq_freeze_lock);
> if (queue_is_mq(q))
> blk_mq_run_hw_queues(q, false);
> + } else {
> + mutex_unlock(&q->mq_freeze_lock);
> }
> }
> EXPORT_SYMBOL_GPL(blk_freeze_queue_start);
> @@ -198,14 +199,14 @@ EXPORT_SYMBOL_GPL(blk_mq_freeze_queue);
>
> void blk_mq_unfreeze_queue(struct request_queue *q)
> {
> - int freeze_depth;
> -
> - freeze_depth = atomic_dec_return(&q->mq_freeze_depth);
> - WARN_ON_ONCE(freeze_depth < 0);
> - if (!freeze_depth) {
> + mutex_lock(&q->mq_freeze_lock);
> + q->mq_freeze_depth--;
> + WARN_ON_ONCE(q->mq_freeze_depth < 0);
> + if (!q->mq_freeze_depth) {
> percpu_ref_resurrect(&q->q_usage_counter);
> wake_up_all(&q->mq_freeze_wq);
> }
> + mutex_unlock(&q->mq_freeze_lock);
> }
> EXPORT_SYMBOL_GPL(blk_mq_unfreeze_queue);
>
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 5c58a3b..64f7683 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -535,7 +535,7 @@ struct request_queue {
>
> struct mutex sysfs_lock;
>
> - atomic_t mq_freeze_depth;
> + int mq_freeze_depth;
>
> #if defined(CONFIG_BLK_DEV_BSG)
> struct bsg_class_device bsg_dev;
> @@ -547,6 +547,11 @@ struct request_queue {
> #endif
> struct rcu_head rcu_head;
> wait_queue_head_t mq_freeze_wq;
> + /*
> + * Protect concurrent access to q_usage_counter by
> + * percpu_ref_kill() and percpu_ref_reinit().
> + */
> + struct mutex mq_freeze_lock;
> struct percpu_ref q_usage_counter;
> struct list_head all_q_node;
Looks fine,
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Thanks,
Ming Lei
On Thu, 2019-04-25 at 03:28 -0700, Bob Liu wrote:
> The following is a description of a hang in blk_mq_freeze_queue_wait().
> The hang happens on attempting to freeze a queue while another task does
> queue unfreeze.
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Hey Jens,
Could you please queue this patch?
As Jinpu mentioned:
"We at 1 & 1 IONOS (former ProfitBricks) have been carried this patch
for some years, it has been running in production for some years too"
Thanks,
Bob
On 4/25/19 6:28 PM, Bob Liu wrote:
> The following is a description of a hang in blk_mq_freeze_queue_wait().
> The hang happens on attempting to freeze a queue while another task does
> queue unfreeze.
>
> The root cause is an incorrect sequence of percpu_ref_resurrect() and
> percpu_ref_kill() and as a result those two can be swapped:
>
> CPU#0 CPU#1
> ---------------- -----------------
> q1 = blk_mq_init_queue(shared_tags)
>
> q2 = blk_mq_init_queue(shared_tags)
> blk_mq_add_queue_tag_set(shared_tags)
> blk_mq_update_tag_set_depth(shared_tags)
> list_for_each_entry()
> blk_mq_freeze_queue(q1)
> > percpu_ref_kill()
> > blk_mq_freeze_queue_wait()
>
> blk_cleanup_queue(q1)
> blk_mq_freeze_queue(q1)
> > percpu_ref_kill()
> ^^^^^^ freeze_depth can't guarantee the order
>
> blk_mq_unfreeze_queue()
> > percpu_ref_resurrect()
>
> > blk_mq_freeze_queue_wait()
> ^^^^^^ Hang here!!!!
>
> This wrong sequence raises kernel warning:
> percpu_ref_kill_and_confirm called more than once on blk_queue_usage_counter_release!
> WARNING: CPU: 0 PID: 11854 at lib/percpu-refcount.c:336 percpu_ref_kill_and_confirm+0x99/0xb0
>
> But the most unpleasant effect is a hang of a blk_mq_freeze_queue_wait(),
> which waits for a zero of a q_usage_counter, which never happens
> because percpu-ref was reinited (instead of being killed) and stays in
> PERCPU state forever.
>
> How to reproduce:
> - "insmod null_blk.ko shared_tags=1 nr_devices=0 queue_mode=2"
> - cpu0: python Script.py 0; taskset the corresponding process running on cpu0
> - cpu1: python Script.py 1; taskset the corresponding process running on cpu1
>
> Script.py:
> ------
> #!/usr/bin/python3
>
> import os
> import sys
>
> while True:
> on = "echo 1 > /sys/kernel/config/nullb/%s/power" % sys.argv[1]
> off = "echo 0 > /sys/kernel/config/nullb/%s/power" % sys.argv[1]
> os.system(on)
> os.system(off)
> ------
>
> This bug was first reported and fixed by Roman, previous discussion:
> [1] Message id: 1443287365-4244-7-git-send-email-akinobu.mita@gmail.com
> [2] Message id: 1443563240-29306-6-git-send-email-tj@kernel.org
> [3] https://patchwork.kernel.org/patch/9268199/
>
> Signed-off-by: Bob Liu <bob.liu@oracle.com>
> ---
> v4:
> - Update commit log
> v3:
> - rebase to v5.1
> v2:
> - forgotten hunk from local repo
> - minor tweaks in the commit message
> ---
> block/blk-core.c | 3 ++-
> block/blk-mq.c | 19 ++++++++++---------
> include/linux/blkdev.h | 7 ++++++-
> 3 files changed, 18 insertions(+), 11 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index a55389b..fb97497 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -433,7 +433,7 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
> smp_rmb();
>
> wait_event(q->mq_freeze_wq,
> - (atomic_read(&q->mq_freeze_depth) == 0 &&
> + (!q->mq_freeze_depth &&
> (pm || (blk_pm_request_resume(q),
> !blk_queue_pm_only(q)))) ||
> blk_queue_dying(q));
> @@ -523,6 +523,7 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
> spin_lock_init(&q->queue_lock);
>
> init_waitqueue_head(&q->mq_freeze_wq);
> + mutex_init(&q->mq_freeze_lock);
>
> /*
> * Init percpu_ref in atomic mode so that it's faster to shutdown.
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index a935483..373af60 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -143,13 +143,14 @@ void blk_mq_in_flight_rw(struct request_queue *q, struct hd_struct *part,
>
> void blk_freeze_queue_start(struct request_queue *q)
> {
> - int freeze_depth;
> -
> - freeze_depth = atomic_inc_return(&q->mq_freeze_depth);
> - if (freeze_depth == 1) {
> + mutex_lock(&q->mq_freeze_lock);
> + if (++q->mq_freeze_depth == 1) {
> percpu_ref_kill(&q->q_usage_counter);
> + mutex_unlock(&q->mq_freeze_lock);
> if (queue_is_mq(q))
> blk_mq_run_hw_queues(q, false);
> + } else {
> + mutex_unlock(&q->mq_freeze_lock);
> }
> }
> EXPORT_SYMBOL_GPL(blk_freeze_queue_start);
> @@ -198,14 +199,14 @@ EXPORT_SYMBOL_GPL(blk_mq_freeze_queue);
>
> void blk_mq_unfreeze_queue(struct request_queue *q)
> {
> - int freeze_depth;
> -
> - freeze_depth = atomic_dec_return(&q->mq_freeze_depth);
> - WARN_ON_ONCE(freeze_depth < 0);
> - if (!freeze_depth) {
> + mutex_lock(&q->mq_freeze_lock);
> + q->mq_freeze_depth--;
> + WARN_ON_ONCE(q->mq_freeze_depth < 0);
> + if (!q->mq_freeze_depth) {
> percpu_ref_resurrect(&q->q_usage_counter);
> wake_up_all(&q->mq_freeze_wq);
> }
> + mutex_unlock(&q->mq_freeze_lock);
> }
> EXPORT_SYMBOL_GPL(blk_mq_unfreeze_queue);
>
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 5c58a3b..64f7683 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -535,7 +535,7 @@ struct request_queue {
>
> struct mutex sysfs_lock;
>
> - atomic_t mq_freeze_depth;
> + int mq_freeze_depth;
>
> #if defined(CONFIG_BLK_DEV_BSG)
> struct bsg_class_device bsg_dev;
> @@ -547,6 +547,11 @@ struct request_queue {
> #endif
> struct rcu_head rcu_head;
> wait_queue_head_t mq_freeze_wq;
> + /*
> + * Protect concurrent access to q_usage_counter by
> + * percpu_ref_kill() and percpu_ref_reinit().
> + */
> + struct mutex mq_freeze_lock;
> struct percpu_ref q_usage_counter;
> struct list_head all_q_node;
>
> --
> 2.9.5
>