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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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@I-love.SAKURA.ne.jp>
> Cc: Vegard Nossum <vegard.nossum@oracle.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Mandeep Singh Baines <msb@chromium.org>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

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@linux.vnet.ibm.com>

> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> ---
>  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
>