Re: 2.6.26.3 mount process looping on ext3 rw remount

[Andrew Morton]

On Sat, 23 Aug 2008 10:42:28 +0200 Marc Haber <mh+linux-kernel@xxxxxxxxxxxx> wrote:

> Hi,
> 
> on my laptop, I am a heavy user of laptop-mode and suspend-to-disk.
> Sometimes (maybe once out of ten), when disconnecting or connecting
> external power, I have a mount process called by laptopmode busy
> looping and taking all available CPU:
> 
>   ______acpid,4680 -c /etc/acpi/events
>   ___   ______lm_ac_adapter.s,29317 /etc/acpi/actions/lm_ac_adapter.sh
>   ___       ______laptop_mode,29318 /usr/sbin/laptop_mode auto
>   ___           ______laptop-mode,29386 /usr/share/laptop-mode-tools/modules/laptop-mode
>   ___               ______laptop-mode,29390 /usr/share/laptop-mode-tools/modules/laptop-mode
>   ___                   ______laptop-mode,29409 /usr/share/laptop-mode-tools/modules/laptop-mode
>   ___                       ______mount,29410 /dev/mapper/usr /mnt/usr -t ext3 -o remount,rw,commit=600
> 
> this is what top says
> Cpu(s):  2.7%us, 96.0%sy,  0.0%ni,  0.0%id,  0.0%wa,  1.3%hi,  0.0%si, 0.0%st
> 29410 root      20   0  2068  684  564 R 92.7  0.0  87:37.71 mount
> 
> The mount process does not react to SIGKILL, stracing the looping
> process doesn't give any output, and the strace gets stuck and does
> not react to Ctrl-C. A SIGKILL works for the strace process, though.
> 
> To me as a layman this looks like the mount process gets stuck
> somewhere in kernel land. I currently have the issue with 2.6.26.3.
> 
> In the situation of plugging and/or unplugging the power, the notebook
> used to completely freeze in the time when 2.6.24 and 2.6.25.$SMALL
> were in use, with 2.6.25.$HIGH and 2.6.26 I haven't hat these freezes
> any more. However, I am now plagued with the hanging mount processes.

Yes, it's hung in the kernel.

Please try to get a kernel profile while it's happening.  oprofile
maybe, or just the plain old timer-based profiler.  There's some info
in Documentation/basic_profiling.txt.

> Any ideas?

The profile will tell us where it got stuck.


Actually, a simple alternative is to hit sysrq-P five or ten times. 
Most of the resulting stack traces will point back at where the CPU is
stuck.

This gets a bit hit-or-miss if you have multiple CPUs, because the
sysrq-p trace can land on the wrong CPU.  We recently added a sysrq-l
which will generate a trace on all CPUS.

I think we might recently have broken the sysrq output: some info which
should be coming out isn't.  Altering the logging priority (dmesg -n 7)
might help with that.

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