[PATCH V2] kernel/hung_task.c: Force console verbose before panic

["Liu, Chuansheng" <chuansheng.liu@intel.com>]

Based on patch commit 401c636a0eeb ("kernel/hung_task.c:
show all hung tasks before panic"), we could get the
call stack of hung task.

However, if the console loglevel is not high, we still
can not see the useful panic information in practice,
and in most cases users don't set console loglevel to
high level.

This patch is to force console verbose before system
panic, so that the real useful information can be seen
in the console, instead of being like the following,
which doesn't have hung task information.

[  246.916600] INFO: task init:1 blocked for more than 120 seconds.
[  246.922320]       Tainted: G     U  W         4.19.0-quilt-2e5dc0ac-g51b6c21d76cc #1
[  246.926790] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  246.932553] Kernel panic - not syncing: hung_task: blocked tasks
[  246.938503] CPU: 2 PID: 479 Comm: khungtaskd Tainted: G     U  W         4.19.0-quilt-2e5dc0ac-g51b6c21d76cc #1
[  246.990266] Call Trace:
[  246.991707]  dump_stack+0x4f/0x65
[  246.993710]  panic+0xde/0x231
[  246.995445]  watchdog+0x290/0x410
[  246.997390]  kthread+0x12c/0x150
[  246.999301]  ? reset_hung_task_detector+0x20/0x20
[  247.004825]  ? kthread_create_worker_on_cpu+0x70/0x70
[  247.007735]  ret_from_fork+0x35/0x40
[  247.010280] reboot: panic mode set: p,w
[  247.012619] Kernel Offset: 0x34000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)

V1: console_verbose() is used instead of ignore_loglevel,
    suggested by Sergey.
    Tweak the function check_hung_task() suggested by
    Tetsuo to make code more readable.

V2: Petr suggests to make 2 patches:
    One is addressing console_verbose() only, which is
    this patch's aim.
    The other is about forcing sched_show_task() during
    panic, which needs more discussion yet.

Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Petr Mladek <pmladek@suse.com>
Signed-off-by: Chuansheng Liu <chuansheng.liu@intel.com>
---
 kernel/hung_task.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index cb8e3e8..85af0cd 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -112,8 +112,11 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)

        trace_sched_process_hang(t);

-       if (!sysctl_hung_task_warnings && !sysctl_hung_task_panic)
-               return;
+       if (sysctl_hung_task_panic) {
+               console_verbose();
+               hung_task_show_lock = true;
+               hung_task_call_panic = true;
+       }

        /*
         * Ok, the task did not get scheduled for more than 2 minutes,
@@ -135,11 +138,6 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
        }

        touch_nmi_watchdog();
-
-       if (sysctl_hung_task_panic) {
-               hung_task_show_lock = true;
-               hung_task_call_panic = true;
-       }
 }

 /*
--
2.7.4