Re: [PATCH] locking/hung_task: Show all hung tasks before panic

[Dmitry Vyukov]

On Mon, Apr 2, 2018 at 5:35 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Apr 02, 2018 at 11:12:04PM +0900, Tetsuo Handa wrote:
>> When we get a hung task it can often be valuable to see _all_ the hung
>> tasks on the system before calling panic().
>>
>> Quoting from https://syzkaller.appspot.com/text?tag=CrashReport&id=5412451675799552
>> ----------------------------------------
>> INFO: task syz-executor3:13421 blocked for more than 120 seconds.
>>       Not tainted 4.16.0-rc7+ #9
>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> syz-executor3   D24672 13421   4481 0x00000004
>> 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
>>  __rwsem_down_read_failed_common kernel/locking/rwsem-xadd.c:269 [inline]
>>  rwsem_down_read_failed+0x401/0x6e0 kernel/locking/rwsem-xadd.c:286
>>  call_rwsem_down_read_failed+0x18/0x30 arch/x86/lib/rwsem.S:94
>>  __down_read arch/x86/include/asm/rwsem.h:83 [inline]
>>  down_read+0xa4/0x150 kernel/locking/rwsem.c:26
>>  __get_super.part.9+0x1d3/0x280 fs/super.c:663
>>  __get_super include/linux/spinlock.h:310 [inline]
>>  get_super+0x2d/0x40 fs/super.c:692
>>  fsync_bdev+0x19/0x80 fs/block_dev.c:468
>>  invalidate_partition+0x35/0x60 block/genhd.c:1566
>>  drop_partitions.isra.12+0xcd/0x1d0 block/partition-generic.c:440
>>  rescan_partitions+0x72/0x900 block/partition-generic.c:513
>>  __blkdev_reread_part+0x15f/0x1e0 block/ioctl.c:173
>>  blkdev_reread_part+0x26/0x40 block/ioctl.c:193
>>  loop_reread_partitions+0x12f/0x1a0 drivers/block/loop.c:619
>>  loop_set_status+0x9bb/0xf60 drivers/block/loop.c:1161
>>  loop_set_status64+0x9d/0x110 drivers/block/loop.c:1271
>>  lo_ioctl+0xd86/0x1b70 drivers/block/loop.c:1381
>> (...snipped...)
>> Showing all locks held in the system:
>> (...snipped...)
>> 3 locks held by syz-executor3/13421:
>>  #0:  (&lo->lo_ctl_mutex/1){+.+.}, at: [<00000000834f78af>] lo_ioctl+0x8b/0x1b70 drivers/block/loop.c:1355 /* mutex_lock_nested(&lo->lo_ctl_mutex, 1); */
>>  #1:  (&bdev->bd_mutex){+.+.}, at: [<0000000003605603>] blkdev_reread_part+0x1e/0x40 block/ioctl.c:192
>>  #2:  (&type->s_umount_key#77){.+.+}, at: [<0000000077701649>] __get_super.part.9+0x1d3/0x280 fs/super.c:663 /* down_read(&sb->s_umount); */
>> (...snipped...)
>> 2 locks held by syz-executor0/13428:
>>  #0:  (&type->s_umount_key#76/1){+.+.}, at: [<00000000d25ba33a>] alloc_super fs/super.c:211 [inline]
>>  #0:  (&type->s_umount_key#76/1){+.+.}, at: [<00000000d25ba33a>] sget_userns+0x3a1/0xe40 fs/super.c:502 /* down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING); */
>>  #1:  (&lo->lo_ctl_mutex/1){+.+.}, at: [<00000000834f78af>] lo_ioctl+0x8b/0x1b70 drivers/block/loop.c:1355 /* mutex_lock_nested(&lo->lo_ctl_mutex, 1); */
>> ----------------------------------------
>>
>> In addition to showing hashed address of lock instances, it would be
>> nice if trace of 13428 is printed as well as 13421.
>>
>> Showing hung tasks up to /proc/sys/kernel/hung_task_warnings could delay
>> calling panic() but normally there should not be so many hung tasks.
>>
>> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
>> Cc: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Mandeep Singh Baines <msb@xxxxxxxxxxxx>
>> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>
> I just know that I am going to regret this the first time this happens
> on a low-speed console port, but...
>
> Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>


Thanks! I think getting these last bits of debugging tools to be more
useful is very important in the context of syzbot. So:

Acked-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>




>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> ---
>>  kernel/hung_task.c | 11 +++++++----
>>  1 file changed, 7 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
>> index 751593e..32b4794 100644
>> --- a/kernel/hung_task.c
>> +++ b/kernel/hung_task.c
>> @@ -44,6 +44,7 @@
>>
>>  static int __read_mostly did_panic;
>>  static bool hung_task_show_lock;
>> +static bool hung_task_call_panic;
>>
>>  static struct task_struct *watchdog_task;
>>
>> @@ -127,10 +128,8 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
>>       touch_nmi_watchdog();
>>
>>       if (sysctl_hung_task_panic) {
>> -             if (hung_task_show_lock)
>> -                     debug_show_all_locks();
>> -             trigger_all_cpu_backtrace();
>> -             panic("hung_task: blocked tasks");
>> +             hung_task_show_lock = true;
>> +             hung_task_call_panic = true;
>>       }
>>  }
>>
>> @@ -193,6 +192,10 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
>>       rcu_read_unlock();
>>       if (hung_task_show_lock)
>>               debug_show_all_locks();
>> +     if (hung_task_call_panic) {
>> +             trigger_all_cpu_backtrace();
>> +             panic("hung_task: blocked tasks");
>> +     }
>>  }
>>
>>  static long hung_timeout_jiffies(unsigned long last_checked,
>> --
>> 1.8.3.1
>>
>