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SubjectRe: [PATCH] oom killer: break from infinite loop
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@gmail.com> 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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