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

[Michal Hocko]

On Fri 28-10-11 15:23:21, Andrew Morton wrote:
> On Tue, 27 Sep 2011 10:01:47 +0200
> Michal Hocko <mhocko@xxxxxxx> 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.
> > 
> > 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 or we always check for pending signals after
> > we return from try_to_freeze (e.g. in lguest).
> > 
> > Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> > Reported-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx>
> > Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > Acked-by: Rafael J. Wysocki <rjw@xxxxxxx>
> > Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> > ---
> >  mm/oom_kill.c |    6 ++++++
> >  1 files changed, 6 insertions(+), 0 deletions(-)
> > 
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index 626303b..c419a7e 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -32,6 +32,7 @@
> >  #include <linux/mempolicy.h>
> >  #include <linux/security.h>
> >  #include <linux/ptrace.h>
> > +#include <linux/freezer.h>
> >  
> >  int sysctl_panic_on_oom;
> >  int sysctl_oom_kill_allocating_task;
> > @@ -451,10 +452,15 @@ static int oom_kill_task(struct task_struct *p, struct mem_cgroup *mem)
> >  				task_pid_nr(q), q->comm);
> >  			task_unlock(q);
> >  			force_sig(SIGKILL, q);
> > +
> > +			if (frozen(q))
> > +				thaw_process(q);
> >  		}
> >  
> >  	set_tsk_thread_flag(p, TIF_MEMDIE);
> >  	force_sig(SIGKILL, p);
> > +	if (frozen(p))
> > +		thaw_process(p);
> >  
> >  	return 0;
> >  }
> 
> I'm not sure this is 1000% correct.  Perhaps there's a conceivable
> window after the "if (frozen)" test where the task can flip itself into
> the frozen state.

Yes and David's patch
(oom-thaw-threads-if-oom-killed-thread-is-frozen-before-deferring.patch)
is much better in that regards. So we should go with the other patch.

> 
> thaw_process() itself appears to be callable regardless of the frozen
> state and will do the right thing under the right lock.  So this code
> would be safer, correcter and slower if it unconditionally called
> thaw_process().
> 
> I'm sure it doesn't matter though ;)
> 
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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