linux-kernel - Re: [printk] fbc14616f4: BUG:kernel_reboot-without-warning_in_test

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Date:   Fri, 31 Mar 2017 23:47:30 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Ye Xiaolong <xiaolong.ye@...el.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Petr Mladek <pmladek@...e.com>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Eric Biederman <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Pavel Machek <pavel@....cz>,
        Len Brown <len.brown@...el.com>, linux-kernel@...r.kernel.org,
        lkp@...org
Subject: Re: [printk]  fbc14616f4:
 BUG:kernel_reboot-without-warning_in_test_stage

On (03/31/17 14:39), Ye Xiaolong wrote:
> On 03/31, Sergey Senozhatsky wrote:
> >On (03/31/17 11:35), Sergey Senozhatsky wrote:
> >[..]
> >> > [   21.009531] VFS: Warning: trinity-c2 using old stat() call. Recompile your binary.
> >> > [   21.148898] VFS: Warning: trinity-c0 using old stat() call. Recompile your binary.
> >> > [   22.298208] warning: process `trinity-c2' used the deprecated sysctl system call with 
> >> > 
> >> > Elapsed time: 310
> >> > BUG: kernel reboot-without-warning in test stage
> >> 
> >> so as far as I understand, this is the "missing kernel messages"
> >> type of bug report. a worst case scenario.
> >
> >panic() should have called console_flush_on_panic(), which sould have
> >flushed the messages regardless the printk_kthread state. so it probably
> >was not panic() that rebooted the kernel. (probably).
> >
> >kernel_restart() and kernel_halt() have pr_emerg() messages, printk switches
> >to printk_emergency mode the first time it sees EMERG level message. (may be
> >we switch to late).
> >
> >on the other hand, there is a emergency_restart(), where we don't switch
> >to printk_emergency mode and don't flush the existing kernel messages.
> >there is a bunch of places that call emergency_restart(), including sysrq.
> >
> >may I ask you, how do you usually restart the vm after the test?
> >`echo X > /proc/sysrq-trigger'?
> 
> Yes.
> 
> >
> >does this patch make it any better?
> 
> I am trying it and will post the result once I get it.


... I'd also probably add pr_emerg() print-out to emergency_restart(),
the same way kernel_restart()/kernel_halt()/kernel_power_off() do.

for those cases when emergency_restart() is called with printk in
kthreaded mode, not in emergency mode.

---

diff --git a/kernel/reboot.c b/kernel/reboot.c
index e4ced883d8de..5bce29da913b 100644
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -60,6 +60,7 @@ void (*pm_power_off_prepare)(void);
  */
 void emergency_restart(void)
 {
+	pr_emerg("Emergency restart\n");
 	kmsg_dump(KMSG_DUMP_EMERG);
 	machine_emergency_restart();
 }