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

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

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