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Subject[PATCH] locking/hung_task: Show all hung tasks before panic
Date
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>
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
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