Re: INFO: task hung in wb_shutdown (2)
On 5/1/18 4:14 PM, Tetsuo Handa wrote:
>>From 1b90d7f71d60e743c69cdff3ba41edd1f9f86f93 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa
> Date: Wed, 2 May 2018 07:07:55 +0900
> Subject: [PATCH v2] bdi: wake up concurrent wb_shutdown() callers.
>
> syzbot is reporting hung tasks at wait_on_bit(WB_shutting_down) in
> wb_shutdown() [1]. This seems to be because commit 5318ce7d46866e1d ("bdi:
> Shutdown writeback on all cgwbs in cgwb_bdi_destroy()") forgot to call
> wake_up_bit(WB_shutting_down) after clear_bit(WB_shutting_down).
>
> Introduce a helper function clear_and_wake_up_bit() and use it, in order
> to avoid similar errors in future.
Queued up, thanks Tetsuo!
--
Jens Axboe
Re: INFO: task hung in wb_shutdown (2)
On Wed 02-05-18 07:14:51, Tetsuo Handa wrote:
> >From 1b90d7f71d60e743c69cdff3ba41edd1f9f86f93 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa
> Date: Wed, 2 May 2018 07:07:55 +0900
> Subject: [PATCH v2] bdi: wake up concurrent wb_shutdown() callers.
>
> syzbot is reporting hung tasks at wait_on_bit(WB_shutting_down) in
> wb_shutdown() [1]. This seems to be because commit 5318ce7d46866e1d ("bdi:
> Shutdown writeback on all cgwbs in cgwb_bdi_destroy()") forgot to call
> wake_up_bit(WB_shutting_down) after clear_bit(WB_shutting_down).
>
> Introduce a helper function clear_and_wake_up_bit() and use it, in order
> to avoid similar errors in future.
>
> [1]
> https://syzkaller.appspot.com/bug?id=b297474817af98d5796bc544e1bb806fc3da0e5e
>
> Signed-off-by: Tetsuo Handa
> Reported-by: syzbot
> Fixes: 5318ce7d46866e1d ("bdi: Shutdown writeback on all cgwbs in
> cgwb_bdi_destroy()")
> Cc: Tejun Heo
> Cc: Jan Kara
> Cc: Jens Axboe
> Suggested-by: Linus Torvalds
Thanks for debugging this and for the fix Tetsuo! The patch looks good to
me. You can add:
Reviewed-by: Jan Kara
Honza
> ---
> include/linux/wait_bit.h | 17 +
> mm/backing-dev.c | 2 +-
> 2 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/wait_bit.h b/include/linux/wait_bit.h
> index 9318b21..2b0072f 100644
> --- a/include/linux/wait_bit.h
> +++ b/include/linux/wait_bit.h
> @@ -305,4 +305,21 @@ struct wait_bit_queue_entry {
> __ret; \
> })
>
> +/**
> + * clear_and_wake_up_bit - clear a bit and wake up anyone waiting on that bit
> + *
> + * @bit: the bit of the word being waited on
> + * @word: the word being waited on, a kernel virtual address
> + *
> + * You can use this helper if bitflags are manipulated atomically rather than
> + * non-atomically under a lock.
> + */
> +static inline void clear_and_wake_up_bit(int bit, void *word)
> +{
> + clear_bit_unlock(bit, word);
> + /* See wake_up_bit() for which memory barrier you need to use. */
> + smp_mb__after_atomic();
> + wake_up_bit(word, bit);
> +}
> +
> #endif /* _LINUX_WAIT_BIT_H */
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 023190c..fa5e6d7 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -383,7 +383,7 @@ static void wb_shutdown(struct bdi_writeback *wb)
>* the barrier provided by test_and_clear_bit() above.
>*/
> smp_wmb();
> - clear_bit(WB_shutting_down, &wb->state);
> + clear_and_wake_up_bit(WB_shutting_down, &wb->state);
> }
>
> static void wb_exit(struct bdi_writeback *wb)
> --
> 1.8.3.1
--
Jan Kara
SUSE Labs, CR
Re: INFO: task hung in wb_shutdown (2)
>From 1b90d7f71d60e743c69cdff3ba41edd1f9f86f93 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa
Date: Wed, 2 May 2018 07:07:55 +0900
Subject: [PATCH v2] bdi: wake up concurrent wb_shutdown() callers.
syzbot is reporting hung tasks at wait_on_bit(WB_shutting_down) in
wb_shutdown() [1]. This seems to be because commit 5318ce7d46866e1d ("bdi:
Shutdown writeback on all cgwbs in cgwb_bdi_destroy()") forgot to call
wake_up_bit(WB_shutting_down) after clear_bit(WB_shutting_down).
Introduce a helper function clear_and_wake_up_bit() and use it, in order
to avoid similar errors in future.
[1]
https://syzkaller.appspot.com/bug?id=b297474817af98d5796bc544e1bb806fc3da0e5e
Signed-off-by: Tetsuo Handa
Reported-by: syzbot
Fixes: 5318ce7d46866e1d ("bdi: Shutdown writeback on all cgwbs in
cgwb_bdi_destroy()")
Cc: Tejun Heo
Cc: Jan Kara
Cc: Jens Axboe
Suggested-by: Linus Torvalds
---
include/linux/wait_bit.h | 17 +
mm/backing-dev.c | 2 +-
2 files changed, 18 insertions(+), 1 deletion(-)
diff --git a/include/linux/wait_bit.h b/include/linux/wait_bit.h
index 9318b21..2b0072f 100644
--- a/include/linux/wait_bit.h
+++ b/include/linux/wait_bit.h
@@ -305,4 +305,21 @@ struct wait_bit_queue_entry {
__ret; \
})
+/**
+ * clear_and_wake_up_bit - clear a bit and wake up anyone waiting on that bit
+ *
+ * @bit: the bit of the word being waited on
+ * @word: the word being waited on, a kernel virtual address
+ *
+ * You can use this helper if bitflags are manipulated atomically rather than
+ * non-atomically under a lock.
+ */
+static inline void clear_and_wake_up_bit(int bit, void *word)
+{
+ clear_bit_unlock(bit, word);
+ /* See wake_up_bit() for which memory barrier you need to use. */
+ smp_mb__after_atomic();
+ wake_up_bit(word, bit);
+}
+
#endif /* _LINUX_WAIT_BIT_H */
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 023190c..fa5e6d7 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -383,7 +383,7 @@ static void wb_shutdown(struct bdi_writeback *wb)
* the barrier provided by test_and_clear_bit() above.
*/
smp_wmb();
- clear_bit(WB_shutting_down, &wb->state);
+ clear_and_wake_up_bit(WB_shutting_down, &wb->state);
}
static void wb_exit(struct bdi_writeback *wb)
--
1.8.3.1
Re: INFO: task hung in wb_shutdown (2)
On 5/1/18 10:06 AM, Linus Torvalds wrote: > On Tue, May 1, 2018 at 3:27 AM Tetsuo Handa < > penguin-ker...@i-love.sakura.ne.jp> wrote: > >> Can you review this patch? syzbot has hit this bug for nearly 4000 times > but >> is still unable to find a reproducer. Therefore, the only way to test > would be >> to apply this patch upstream and test whether the problem is solved. > > Looks ok to me, except: > >>> smp_wmb(); >>> clear_bit(WB_shutting_down, &wb->state); >>> + smp_mb(); /* advised by wake_up_bit() */ >>> + wake_up_bit(&wb->state, WB_shutting_down); > > This whole sequence really should just be a pattern with a helper function. > > And honestly, the pattern probably *should* be > > clear_bit_unlock(bit, &mem); > smp_mb__after_atomic() > wake_up_bit(&mem, bit); > > which looks like it is a bit cleaner wrt memory ordering rules. Agree, that construct looks saner than introducing a "random" smp_mb(). As a pattern helper, should probably be introduced after the fact. -- Jens Axboe
Re: INFO: task hung in wb_shutdown (2)
On 5/1/18 4:27 AM, Tetsuo Handa wrote: > Tejun, Jan, Jens, > > Can you review this patch? syzbot has hit this bug for nearly 4000 times but > is still unable to find a reproducer. Therefore, the only way to test would be > to apply this patch upstream and test whether the problem is solved. I'll review it today. -- Jens Axboe
Re: INFO: task hung in wb_shutdown (2)
On Tue, May 1, 2018 at 3:27 AM Tetsuo Handa < penguin-ker...@i-love.sakura.ne.jp> wrote: > Can you review this patch? syzbot has hit this bug for nearly 4000 times but > is still unable to find a reproducer. Therefore, the only way to test would be > to apply this patch upstream and test whether the problem is solved. Looks ok to me, except: > > smp_wmb(); > > clear_bit(WB_shutting_down, &wb->state); > > + smp_mb(); /* advised by wake_up_bit() */ > > + wake_up_bit(&wb->state, WB_shutting_down); This whole sequence really should just be a pattern with a helper function. And honestly, the pattern probably *should* be clear_bit_unlock(bit, &mem); smp_mb__after_atomic() wake_up_bit(&mem, bit); which looks like it is a bit cleaner wrt memory ordering rules. Linus
Re: INFO: task hung in wb_shutdown (2)
Tejun, Jan, Jens,
Can you review this patch? syzbot has hit this bug for nearly 4000 times but
is still unable to find a reproducer. Therefore, the only way to test would be
to apply this patch upstream and test whether the problem is solved.
On 2018/04/24 21:19, Tetsuo Handa wrote:
>>From 39ed6be8a2c12dfe54feaa5abbc2ec46103022bf Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa
> Date: Tue, 24 Apr 2018 11:59:08 +0900
> Subject: [PATCH] bdi: wake up concurrent wb_shutdown() callers.
>
> syzbot is reporting hung tasks at wait_on_bit(WB_shutting_down) in
> wb_shutdown() [1]. This might be because commit 5318ce7d46866e1d ("bdi:
> Shutdown writeback on all cgwbs in cgwb_bdi_destroy()") forgot to call
> wake_up_bit(WB_shutting_down) after clear_bit(WB_shutting_down).
>
> [1]
> https://syzkaller.appspot.com/bug?id=b297474817af98d5796bc544e1bb806fc3da0e5e
>
> Signed-off-by: Tetsuo Handa
> Reported-by: syzbot
> Fixes: 5318ce7d46866e1d ("bdi: Shutdown writeback on all cgwbs in
> cgwb_bdi_destroy()")
> Cc: Tejun Heo
> Cc: Jan Kara
> Cc: Jens Axboe
> ---
> mm/backing-dev.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 023190c..dadac99 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -384,6 +384,8 @@ static void wb_shutdown(struct bdi_writeback *wb)
>*/
> smp_wmb();
> clear_bit(WB_shutting_down, &wb->state);
> + smp_mb(); /* advised by wake_up_bit() */
> + wake_up_bit(&wb->state, WB_shutting_down);
> }
>
> static void wb_exit(struct bdi_writeback *wb)
>
Re: INFO: task hung in wb_shutdown (2)
>From 39ed6be8a2c12dfe54feaa5abbc2ec46103022bf Mon Sep 17 00:00:00 2001
From: Tetsuo Handa
Date: Tue, 24 Apr 2018 11:59:08 +0900
Subject: [PATCH] bdi: wake up concurrent wb_shutdown() callers.
syzbot is reporting hung tasks at wait_on_bit(WB_shutting_down) in
wb_shutdown() [1]. This might be because commit 5318ce7d46866e1d ("bdi:
Shutdown writeback on all cgwbs in cgwb_bdi_destroy()") forgot to call
wake_up_bit(WB_shutting_down) after clear_bit(WB_shutting_down).
[1]
https://syzkaller.appspot.com/bug?id=b297474817af98d5796bc544e1bb806fc3da0e5e
Signed-off-by: Tetsuo Handa
Reported-by: syzbot
Fixes: 5318ce7d46866e1d ("bdi: Shutdown writeback on all cgwbs in
cgwb_bdi_destroy()")
Cc: Tejun Heo
Cc: Jan Kara
Cc: Jens Axboe
---
mm/backing-dev.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 023190c..dadac99 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -384,6 +384,8 @@ static void wb_shutdown(struct bdi_writeback *wb)
*/
smp_wmb();
clear_bit(WB_shutting_down, &wb->state);
+ smp_mb(); /* advised by wake_up_bit() */
+ wake_up_bit(&wb->state, WB_shutting_down);
}
static void wb_exit(struct bdi_writeback *wb)
--
1.8.3.1
INFO: task hung in wb_shutdown (2)
Hello,
syzbot hit the following crash on upstream commit
3eb2ce825ea1ad89d20f7a3b5780df850e4be274 (Sun Mar 25 22:44:30 2018 +)
Linux 4.16-rc7
syzbot dashboard link:
https://syzkaller.appspot.com/bug?extid=c0cf869505e03bdf1a24
So far this crash happened 179 times on upstream.
Unfortunately, I don't have any reproducer for this crash yet.
Raw console output:
https://syzkaller.appspot.com/x/log.txt?id=4738516814659584
Kernel config:
https://syzkaller.appspot.com/x/.config?id=-8440362230543204781
compiler: gcc (GCC) 7.1.1 20170620
IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+c0cf869505e03bdf1...@syzkaller.appspotmail.com
It will help syzbot understand when the bug is fixed. See footer for
details.
If you forward the report, please keep this part and the footer.
unregister_netdevice: waiting for lo to become free. Usage count = 1
unregister_netdevice: waiting for lo to become free. Usage count = 1
unregister_netdevice: waiting for lo to become free. Usage count = 1
unregister_netdevice: waiting for lo to become free. Usage count = 1
unregister_netdevice: waiting for lo to become free. Usage count = 1
INFO: task kworker/0:5:16458 blocked for more than 120 seconds.
Not tainted 4.16.0-rc7+ #368
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/0:5 D20928 16458 2 0x8000
Workqueue: events cgwb_release_workfn
Call Trace:
context_switch kernel/sched/core.c:2862 [inline]
__schedule+0x8fb/0x1ec0 kernel/sched/core.c:3440
schedule+0xf5/0x430 kernel/sched/core.c:3499
bit_wait+0x18/0x90 kernel/sched/wait_bit.c:250
__wait_on_bit+0x88/0x130 kernel/sched/wait_bit.c:51
out_of_line_wait_on_bit+0x204/0x3a0 kernel/sched/wait_bit.c:64
wait_on_bit include/linux/wait_bit.h:84 [inline]
wb_shutdown+0x335/0x430 mm/backing-dev.c:377
cgwb_release_workfn+0x8b/0x61d mm/backing-dev.c:520
process_one_work+0xc47/0x1bb0 kernel/workqueue.c:2113
worker_thread+0x223/0x1990 kernel/workqueue.c:2247
kthread+0x33c/0x400 kernel/kthread.c:238
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:406
Showing all locks held in the system:
3 locks held by kworker/u4:1/21:
#0: ((wq_completion)"%s""netns"){+.+.}, at: []
work_static include/linux/workqueue.h:198 [inline]
#0: ((wq_completion)"%s""netns"){+.+.}, at: []
set_work_data kernel/workqueue.c:619 [inline]
#0: ((wq_completion)"%s""netns"){+.+.}, at: []
set_work_pool_and_clear_pending kernel/workqueue.c:646 [inline]
#0: ((wq_completion)"%s""netns"){+.+.}, at: []
process_one_work+0xb12/0x1bb0 kernel/workqueue.c:2084
#1: (net_cleanup_work){+.+.}, at: [<6c4c2cfd>]
process_one_work+0xb89/0x1bb0 kernel/workqueue.c:2088
#2: (net_mutex){+.+.}, at: [<58427774>] cleanup_net+0x242/0xcb0
net/core/net_namespace.c:484
2 locks held by khungtaskd/869:
#0: (rcu_read_lock){}, at: [<9066e5de>]
check_hung_uninterruptible_tasks kernel/hung_task.c:175 [inline]
#0: (rcu_read_lock){}, at: [<9066e5de>] watchdog+0x1c5/0xd60
kernel/hung_task.c:249
#1: (tasklist_lock){.+.+}, at: [<2117cbd8>]
debug_show_all_locks+0xd3/0x3d0 kernel/locking/lockdep.c:4470
2 locks held by getty/4407:
#0: (&tty->ldisc_sem){}, at: [<15dafb41>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<841085d3>]
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4408:
#0: (&tty->ldisc_sem){}, at: [<15dafb41>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<841085d3>]
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4409:
#0: (&tty->ldisc_sem){}, at: [<15dafb41>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<841085d3>]
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4410:
#0: (&tty->ldisc_sem){}, at: [<15dafb41>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<841085d3>]
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4411:
#0: (&tty->ldisc_sem){}, at: [<15dafb41>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<841085d3>]
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4412:
#0: (&tty->ldisc_sem){}, at: [<15dafb41>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<841085d3>]
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4413:
#0: (&tty->ldisc_sem){}, at: [<15dafb41>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<
