Re: [PATCH 0/3] patches for stop_machine

[Pavel Machek <pavel@ucw.cz>]

Hi!

> Hi Rusty and all,
> 
> This is a proposal of minor improvement for kernel/stop_machine.c
> 
> [PATCH 1/3] stop_machine: short exit path for if we cannot create enough threads
> [PATCH 2/3] stop_machine: add timeout for child thread deployment
> [PATCH 3/3] stop_machine: add stopmachine_timeout sysctl entry
> 
> The main topic is "how about adding timeout for stop_machine?"
> I think it will act as a safety net.
> 
> For example (of silly situation), system can hung with following way:
> 
>   # ./silly.sh
>   run an evil loop task on AP
>    pid 6138's current affinity mask: ff
>    pid 6138's new affinity mask: fe
>    to pretend lock up, chrt -f -p 99 6138
>    loop[6138] is on CPU #4
>   to do stopmachine, try to off #7
>   echo 0 > /sys/devices/system/cpu/cpu7/online
>   (never return)
> 
> After applying patch set here, it can be prevented.
> 
>   # ./silly.sh
>    :
>   echo 0 > /sys/devices/system/cpu/cpu7/online
>   stopmachine: Failed to stop machine in time(5s). Are there any CPUs on file?
>   ./silly.sh: line 22: echo: write error: Device or resource busy
>   offline is failed

I'd expect at least WARN_ON here. -EBUSY is not good enough indication
that one of your cpus is now dead.

							Pavel
-- 
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