Re: [PATCH 1/2] oom: do not live lock on frozen tasks

[David Rientjes]

On Mon, 26 Sep 2011, Rafael J. Wysocki wrote:

> > Konstantin Khlebnikov has reported (https://lkml.org/lkml/2011/8/23/45)
> > that OOM can end up in a live lock if select_bad_process picks up a frozen
> > task.
> > Unfortunately we cannot mark such processes as unkillable to ignore them
> > because we could panic the system even though there is a chance that
> > somebody could thaw the process so we can make a forward process (e.g. a
> > process from another cpuset or with a different nodemask).
> > 
> > Let's thaw an OOM selected frozen process right after we've sent fatal
> > signal from oom_kill_task.
> > Thawing is safe if the frozen task doesn't access any suspended device
> > (e.g. by ioctl) on the way out to the userspace where we handle the
> > signal and die. Note, we are not interested in the kernel threads because
> > they are not oom killable.
> > 
> > Accessing suspended devices by a userspace processes shouldn't be an
> > issue because devices are suspended only after userspace is already
> > frozen and oom is disabled at that time.
> > 
> > run_guest (drivers/lguest/core.c) calls try_to_freeze with an user
> > context but it seems it is able to cope with signals because it
> > explicitly checks for pending signals so we should be safe.
> > 
> > Other than that userspace accesses the fridge only from the
> > signal handling routines so we are able to handle SIGKILL without any
> > negative side effects.
> > 
> > Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> > Reported-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx>
> 
> Acked-by: Rafael J. Wysocki <rjw@xxxxxxx>
> 

Acked-by: David Rientjes <rientjes@xxxxxxxxxx>

Although this still seems to be problematic if the chosen thread gets 
frozen before the SIGKILL can be handled.  We don't have any checks for 
fatal_signal_pending() when freezing threads and waiting for them to exit?

Michal, could you send Andrew your revised patch with all the acked-bys?

Thanks!
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