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    Subject[PATCH 4.17 031/336] kernel/hung_task.c: show all hung tasks before panic
    Date
    4.17-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

    [ Upstream commit 401c636a0eeb0d51862fce222da1bf08e3a0ffd0 ]

    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=5316056503549952
    ----------------------------------------
    INFO: task syz-executor0:6540 blocked for more than 120 seconds.
    Not tainted 4.16.0+ #13
    "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    syz-executor0 D23560 6540 4521 0x80000004
    Call Trace:
    context_switch kernel/sched/core.c:2848 [inline]
    __schedule+0x8fb/0x1ef0 kernel/sched/core.c:3490
    schedule+0xf5/0x430 kernel/sched/core.c:3549
    schedule_preempt_disabled+0x10/0x20 kernel/sched/core.c:3607
    __mutex_lock_common kernel/locking/mutex.c:833 [inline]
    __mutex_lock+0xb7f/0x1810 kernel/locking/mutex.c:893
    mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:908
    lo_ioctl+0x8b/0x1b70 drivers/block/loop.c:1355
    __blkdev_driver_ioctl block/ioctl.c:303 [inline]
    blkdev_ioctl+0x1759/0x1e00 block/ioctl.c:601
    ioctl_by_bdev+0xa5/0x110 fs/block_dev.c:2060
    isofs_get_last_session fs/isofs/inode.c:567 [inline]
    isofs_fill_super+0x2ba9/0x3bc0 fs/isofs/inode.c:660
    mount_bdev+0x2b7/0x370 fs/super.c:1119
    isofs_mount+0x34/0x40 fs/isofs/inode.c:1560
    mount_fs+0x66/0x2d0 fs/super.c:1222
    vfs_kern_mount.part.26+0xc6/0x4a0 fs/namespace.c:1037
    vfs_kern_mount fs/namespace.c:2514 [inline]
    do_new_mount fs/namespace.c:2517 [inline]
    do_mount+0xea4/0x2b90 fs/namespace.c:2847
    ksys_mount+0xab/0x120 fs/namespace.c:3063
    SYSC_mount fs/namespace.c:3077 [inline]
    SyS_mount+0x39/0x50 fs/namespace.c:3074
    do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
    entry_SYSCALL_64_after_hwframe+0x42/0xb7
    (...snipped...)
    Showing all locks held in the system:
    (...snipped...)
    2 locks held by syz-executor0/6540:
    #0: 00000000566d4c39 (&type->s_umount_key#49/1){+.+.}, at: alloc_super fs/super.c:211 [inline]
    #0: 00000000566d4c39 (&type->s_umount_key#49/1){+.+.}, at: sget_userns+0x3b2/0xe60 fs/super.c:502 /* down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING); */
    #1: 0000000043ca8836 (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8b/0x1b70 drivers/block/loop.c:1355 /* mutex_lock_nested(&lo->lo_ctl_mutex, 1); */
    (...snipped...)
    3 locks held by syz-executor7/6541:
    #0: 0000000043ca8836 (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8b/0x1b70 drivers/block/loop.c:1355 /* mutex_lock_nested(&lo->lo_ctl_mutex, 1); */
    #1: 000000007bf3d3f9 (&bdev->bd_mutex){+.+.}, at: blkdev_reread_part+0x1e/0x40 block/ioctl.c:192
    #2: 00000000566d4c39 (&type->s_umount_key#50){.+.+}, at: __get_super.part.10+0x1d3/0x280 fs/super.c:663 /* down_read(&sb->s_umount); */
    ----------------------------------------

    When reporting an AB-BA deadlock like shown above, it would be nice if
    trace of PID=6541 is printed as well as trace of PID=6540 before calling
    panic().

    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.

    Link: http://lkml.kernel.org/r/201804050705.BHE57833.HVFOFtSOMQJFOL@I-love.SAKURA.ne.jp
    Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
    Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Acked-by: Dmitry Vyukov <dvyukov@google.com>
    Cc: Vegard Nossum <vegard.nossum@oracle.com>
    Cc: Mandeep Singh Baines <msb@chromium.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    kernel/hung_task.c | 11 +++++++----
    1 file changed, 7 insertions(+), 4 deletions(-)

    --- a/kernel/hung_task.c
    +++ b/kernel/hung_task.c
    @@ -44,6 +44,7 @@ int __read_mostly sysctl_hung_task_warni

    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_
    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_t
    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,

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