INFO: task hung in wb_shutdown (2)

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
3eb2ce825ea1ad89d20f7a3b5780df850e4be274 (Sun Mar 25 22:44:30 2018 +0000)
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+c0cf86...@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 0x80000000
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: [<00000000c0d07d52>]
work_static include/linux/workqueue.h:198 [inline]
#0: ((wq_completion)"%s""netns"){+.+.}, at: [<00000000c0d07d52>]
set_work_data kernel/workqueue.c:619 [inline]
#0: ((wq_completion)"%s""netns"){+.+.}, at: [<00000000c0d07d52>]
set_work_pool_and_clear_pending kernel/workqueue.c:646 [inline]
#0: ((wq_completion)"%s""netns"){+.+.}, at: [<00000000c0d07d52>]
process_one_work+0xb12/0x1bb0 kernel/workqueue.c:2084
#1: (net_cleanup_work){+.+.}, at: [<000000006c4c2cfd>]
process_one_work+0xb89/0x1bb0 kernel/workqueue.c:2088
#2: (net_mutex){+.+.}, at: [<0000000058427774>] cleanup_net+0x242/0xcb0
net/core/net_namespace.c:484
2 locks held by khungtaskd/869:
#0: (rcu_read_lock){....}, at: [<000000009066e5de>]
check_hung_uninterruptible_tasks kernel/hung_task.c:175 [inline]
#0: (rcu_read_lock){....}, at: [<000000009066e5de>] watchdog+0x1c5/0xd60
kernel/hung_task.c:249
#1: (tasklist_lock){.+.+}, at: [<000000002117cbd8>]
debug_show_all_locks+0xd3/0x3d0 kernel/locking/lockdep.c:4470
2 locks held by getty/4407:
#0: (&tty->ldisc_sem){++++}, at: [<0000000015dafb41>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<00000000841085d3>]
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4408:
#0: (&tty->ldisc_sem){++++}, at: [<0000000015dafb41>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<00000000841085d3>]
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4409:
#0: (&tty->ldisc_sem){++++}, at: [<0000000015dafb41>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<00000000841085d3>]
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4410:
#0: (&tty->ldisc_sem){++++}, at: [<0000000015dafb41>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<00000000841085d3>]
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4411:
#0: (&tty->ldisc_sem){++++}, at: [<0000000015dafb41>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<00000000841085d3>]
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4412:
#0: (&tty->ldisc_sem){++++}, at: [<0000000015dafb41>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<00000000841085d3>]
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4413:
#0: (&tty->ldisc_sem){++++}, at: [<0000000015dafb41>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<00000000841085d3>]
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by kworker/0:5/16458:
#0: ((wq_completion)"events"){+.+.}, at: [<00000000c0d07d52>] work_static
include/linux/workqueue.h:198 [inline]
#0: ((wq_completion)"events"){+.+.}, at: [<00000000c0d07d52>]
set_work_data kernel/workqueue.c:619 [inline]
#0: ((wq_completion)"events"){+.+.}, at: [<00000000c0d07d52>]
set_work_pool_and_clear_pending kernel/workqueue.c:646 [inline]
#0: ((wq_completion)"events"){+.+.}, at: [<00000000c0d07d52>]
process_one_work+0xb12/0x1bb0 kernel/workqueue.c:2084
#1: ((work_completion)(&wb->release_work)){+.+.}, at:
[<000000006c4c2cfd>] process_one_work+0xb89/0x1bb0 kernel/workqueue.c:2088
1 lock held by syz-executor6/23709:
#0: (net_mutex){+.+.}, at: [<00000000843d65a3>] copy_net_ns+0x1f5/0x580
net/core/net_namespace.c:417

=============================================

NMI backtrace for cpu 0
CPU: 0 PID: 869 Comm: khungtaskd Not tainted 4.16.0-rc7+ #368
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x194/0x24d lib/dump_stack.c:53
nmi_cpu_backtrace+0x1d2/0x210 lib/nmi_backtrace.c:103
nmi_trigger_cpumask_backtrace+0x123/0x180 lib/nmi_backtrace.c:62
arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
trigger_all_cpu_backtrace include/linux/nmi.h:138 [inline]
check_hung_task kernel/hung_task.c:132 [inline]
check_hung_uninterruptible_tasks kernel/hung_task.c:190 [inline]
watchdog+0x90c/0xd60 kernel/hung_task.c:249
kthread+0x33c/0x400 kernel/kthread.c:238
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:406
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1 skipped: idling at native_safe_halt+0x6/0x10
arch/x86/include/asm/irqflags.h:54


---
This bug is generated by a dumb bot. It may contain errors.
See https://goo.gl/tpsmEJ for details.
Direct all questions to syzk...@googlegroups.com.

syzbot will keep track of this bug report.
If you forgot to add the Reported-by tag, once the fix for this bug is
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#syz fix: exact-commit-title
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Note: if the crash happens again, it will cause creation of a new bug
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Note: all commands must start from beginning of the line in the email body.

[Tetsuo Handa]

From 39ed6be8a2c12dfe54feaa5abbc2ec46103022bf Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin...@I-love.SAKURA.ne.jp>
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 <penguin...@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+c0cf86...@syzkaller.appspotmail.com>
Fixes: 5318ce7d46866e1d ("bdi: Shutdown writeback on all cgwbs in cgwb_bdi_destroy()")
Cc: Tejun Heo <t...@kernel.org>
Cc: Jan Kara <ja...@suse.cz>
Cc: Jens Axboe <ax...@fb.com>
---
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

[Tetsuo Handa]

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.

[Linus Torvalds]

On Tue, May 1, 2018 at 3:27 AM Tetsuo Handa <
penguin...@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

[Jens Axboe]

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

[Jens Axboe]

Agree, that construct looks saner than introducing a "random"
smp_mb(). As a pattern helper, should probably be introduced
after the fact.

--
Jens Axboe

[Tetsuo Handa]

>From 1b90d7f71d60e743c69cdff3ba41edd1f9f86f93 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin...@I-love.SAKURA.ne.jp>
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 <penguin...@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+c0cf86...@syzkaller.appspotmail.com>
Fixes: 5318ce7d46866e1d ("bdi: Shutdown writeback on all cgwbs in cgwb_bdi_destroy()")
Cc: Tejun Heo <t...@kernel.org>
Cc: Jan Kara <ja...@suse.cz>
Cc: Jens Axboe <ax...@fb.com>

Suggested-by: Linus Torvalds <torv...@linux-foundation.org>
---
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]

On Wed 02-05-18 07:14:51, Tetsuo Handa wrote:
> >From 1b90d7f71d60e743c69cdff3ba41edd1f9f86f93 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin...@I-love.SAKURA.ne.jp>
> 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 <penguin...@I-love.SAKURA.ne.jp>
> Reported-by: syzbot <syzbot+c0cf86...@syzkaller.appspotmail.com>
> Fixes: 5318ce7d46866e1d ("bdi: Shutdown writeback on all cgwbs in cgwb_bdi_destroy()")
> Cc: Tejun Heo <t...@kernel.org>
> Cc: Jan Kara <ja...@suse.cz>
> Cc: Jens Axboe <ax...@fb.com>
> Suggested-by: Linus Torvalds <torv...@linux-foundation.org>

Thanks for debugging this and for the fix Tetsuo! The patch looks good to
me. You can add:

Reviewed-by: Jan Kara <ja...@suse.cz>

Honza

--
Jan Kara <ja...@suse.com>
SUSE Labs, CR

[Jens Axboe]

On 5/1/18 4:14 PM, Tetsuo Handa wrote:
>>From 1b90d7f71d60e743c69cdff3ba41edd1f9f86f93 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin...@I-love.SAKURA.ne.jp>
> 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

[Tetsuo Handa]

OK. Patch is in linux-block.git#for-linus as 8236b0ae31c837d2.

#syz fix: bdi: wake up concurrent wb_shutdown() callers.