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SubjectRe: [kernel.org users] [KORG] Panics on master backend
On 08/25, Peter Zijlstra wrote:
>
> On Wed, 2011-08-24 at 18:08 +0200, Oleg Nesterov wrote:
> > >
> > > static void ttwu_queue(struct task_struct *p, int cpu)
> > > @@ -2705,7 +2703,6 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> > > * this task as prev, wait until its done referencing the task.
> > > */
> > > while (p->on_cpu) {
> > > -#ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
> > > /*
> > > * In case the architecture enables interrupts in
> > > * context_switch(), we cannot busy wait, since that
> > > @@ -2713,11 +2710,11 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> > > * tries to wake up @prev. So bail and do a complete
> > > * remote wakeup.
> > > */
> > > - if (ttwu_activate_remote(p, wake_flags))
> > > + if (cpu == smp_processor_id() &&
> >
> > I think this needs "task_cpu(p) == smp_processor_id()". We can't trust
> > "cpu", task_cpu() was called before ->on_rq check.
>
> Isn't us holding ->pi_lock sufficient to stabilize task_cpu()? If its a
> running task the initial ->state check would have failed,

Of course it is not TASK_RUNNING, but it can be running or not.

> and thus its a
> proper wakeup when we get here and thus ->pi_lock is serializing things.

I am not sure. If ->on_rq is true, we need rq->lock. Say, pull_task() can
change its cpu.

> > --- x/kernel/sched.c
> > +++ x/kernel/sched.c
> > @@ -2694,10 +2694,11 @@ try_to_wake_up(struct task_struct *p, un
> > goto out;
> >
> > success = 1; /* we're going to change ->state */
> > - cpu = task_cpu(p);
> >
> > - if (p->on_rq && ttwu_remote(p, wake_flags))
> > + if (p->on_rq && ttwu_remote(p, wake_flags)) {
> > + cpu = task_cpu(p); /* for ttwu_stat() */
> > goto stat;
> > + }
> >
> > #ifdef CONFIG_SMP
> > /*
>
> Would result in the same problem as below...

I see, thanks.

Yes, ttwu_queue(p, cpu) needs this task_cpu() without CONFIG_SMP.

Oleg.



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