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From: Peter Zijlstra on 24 Mar 2010 11:50
On Mon, 2010-03-15 at 10:10 +0100, Oleg Nesterov wrote:
> static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
> {
> + struct rq *rq = cpu_rq(dead_cpu);
> + int needs_cpu, dest_cpu;
> + unsigned long flags;
> again:
> + local_irq_save(flags);
> +
> + raw_spin_lock(&rq->lock);
> + needs_cpu = (task_cpu(p) == dead_cpu) && (p->state != TASK_WAKING);

^
kernel/sched.c:5445: warning: 'dest_cpu' may be used uninitialized in this function

> + if (needs_cpu)
> + dest_cpu = select_fallback_rq(dead_cpu, p);
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From: Peter Zijlstra on 24 Mar 2010 12:20
On Wed, 2010-03-24 at 17:07 +0100, Oleg Nesterov wrote:
> On 03/24, Peter Zijlstra wrote:
> >
> > On Mon, 2010-03-15 at 10:10 +0100, Oleg Nesterov wrote:
> > > static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
> > > {
> > > + struct rq *rq = cpu_rq(dead_cpu);
> > > + int needs_cpu, dest_cpu;
> > > + unsigned long flags;
> > > again:
> > > + local_irq_save(flags);
> > > +
> > > + raw_spin_lock(&rq->lock);
> > > + needs_cpu = (task_cpu(p) == dead_cpu) && (p->state != TASK_WAKING);
> >
> > ^
> > kernel/sched.c:5445: warning: 'dest_cpu' may be used uninitialized in this function
>
> Hmm. looks like my gcc is more friendly...

Hrm, that and I'm apparently unable to read, it said dest_cpu, not
dead_cpu.. a well, I'll slam an __maybe_unused in.

> OK. certainly I'll send the updated patch, if this series passes
> your review otherwise.

Yeah, you made a few good points in 0/6, am now staring at the code on
how to close those holes, hope to post something sensible soon.
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From: Peter Zijlstra on 26 Mar 2010 05:10
On Wed, 2010-03-24 at 17:33 +0100, Oleg Nesterov wrote:
> On 03/24, Peter Zijlstra wrote:
> >
> > Yeah, you made a few good points in 0/6, am now staring at the code on
> > how to close those holes, hope to post something sensible soon.
>
> Yes, great.
>
> Speaking of 0/6, I forgot to ask a couple more question...
>
> try_to_wake_up() does task_rq_lock() which checks TASK_WAKING. Perhaps
> it shouldn't ? I mean, perhaps try_to_wake_up() can take rq->lock without
> checking task->state. It can never race with the owner of TASK_WAKING,
> before anything else we check "p->state & state".

You're right, but creating a special task_rq_lock() for ttwu() went a
little far, but now that we can remove all that again, this too should
be good again.


> And a stupid question. While doing these changes I was really, really
> puzzled by task_rq_lock() which does
>
> local_irq_save(*flags);
> rq = task_rq(p);
> raw_spin_lock(&rq->lock);
>
> to the point, I even tried to read the comment which says:
>
> Note the ordering: we can safely lookup the task_rq without
> explicitly disabling preemption.
>
> Could you please explain what does this mean? IOW, why can't we do
>
> rq = task_rq(p);
> raw_spin_lock_irqsave(&rq->lock, flags);
>
> instead?

I'm not sure why that is the case, v2.6.14:kernel/sched.c already has
that. Ingo can you remember any reason for this or should we change the
code like Oleg suggests?

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