Re: [PATCH] oom killer: break from infinite loop

[anfei]

On Fri, Mar 26, 2010 at 11:33:56PM +0100, Oleg Nesterov wrote:
> On 03/26, Andrew Morton wrote:
> >
> > On Thu, 25 Mar 2010 00:25:05 +0800
> > Anfei Zhou <anfei.zhou@xxxxxxxxx> wrote:
> >
> > > --- a/mm/oom_kill.c
> > > +++ b/mm/oom_kill.c
> > > @@ -381,6 +381,8 @@ static void dump_header(struct task_struct *p, gfp_t gfp_mask, int order,
> > >   */
> > >  static void __oom_kill_task(struct task_struct *p, int verbose)
> > >  {
> > > +	struct task_struct *t;
> > > +
> > >  	if (is_global_init(p)) {
> > >  		WARN_ON(1);
> > >  		printk(KERN_WARNING "tried to kill init!\n");
> > > @@ -412,6 +414,8 @@ static void __oom_kill_task(struct task_struct *p, int verbose)
> > >  	 */
> > >  	p->rt.time_slice = HZ;
> > >  	set_tsk_thread_flag(p, TIF_MEMDIE);
> > > +	for (t = next_thread(p); t != p; t = next_thread(t))
> > > +		set_tsk_thread_flag(t, TIF_MEMDIE);
> > >
> > >  	force_sig(SIGKILL, p);
> >
> > Don't we need some sort of locking while walking that ring?
> 
> This should be always called under tasklist_lock, I think.
> At least this seems to be true in Linus's tree.
> 
Yes, this function is always called with read_lock(&tasklist_lock), so
it should be okay.

> I'd suggest to do
> 
> 	- set_tsk_thread_flag(p, TIF_MEMDIE);
> 	+ t = p;
> 	+ do {
> 	+	 set_tsk_thread_flag(t, TIF_MEMDIE);
> 	+ } while_each_thread(p, t);
> 
> but this is matter of taste.
> 
Yes, this is better.

> Off-topic, but we shouldn't use force_sig(), SIGKILL doesn't
> need "force" semantics.
> 
This may need a dedicated patch, there are some other places to
force_sig(SIGKILL, ...) too.

> I'd wish I could understand the changelog ;)
> 
Assume thread A and B are in the same group.  If A runs into the oom,
and selects B as the victim, B won't exit because at least in exit_mm(),
it can not get the mm->mmap_sem semaphore which A has already got.  So
no memory is freed, and no other task will be selected to kill.

I formatted the patch for -mm tree as David suggested.


---
 mm/oom_kill.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -418,8 +418,15 @@ static void dump_header(struct task_stru
  */
 static void __oom_kill_task(struct task_struct *p)
 {
+	struct task_struct *t;
+
 	p->rt.time_slice = HZ;
-	set_tsk_thread_flag(p, TIF_MEMDIE);
+
+	t = p;
+	do {
+		set_tsk_thread_flag(t, TIF_MEMDIE);
+	} while_each_thread(p, t);
+
 	force_sig(SIGKILL, p);
 }
 
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