On Thu, Jun 21, 2007 at 10:39:31AM +0200, Ingo Molnar wrote:
>
> * Jarek Poplawski <[EMAIL PROTECTED]> wrote:
>
> > BTW, I've looked a bit at these NMI watchdog traces, and now I'm not
> > even sure it's necessarily the spinlock's problem (but I don't exclude
> > this possibility yet). It seems both processors use task_rq_lock(), so
> > there could be also a problem with that loop. The way the correctness
> > of the taken lock is verified is racy: there is a small probability
> > that if we have taken the wrong lock the check inside the loop is done
> > just before the value is beeing changed elsewhere under the right
> > lock. Another possible problem could be a result of some wrong
> > optimization or wrong propagation of change of this task_rq(p) value.
>
> ok, could you elaborate this in a bit more detail? You say it's racy -
> any correctness bug in task_rq_lock() will cause the kernel to blow up
> in spectacular ways. It's a fairly straightforward loop:
>
> static inline struct rq *__task_rq_lock(struct task_struct *p)
> __acquires(rq->lock)
> {
> struct rq *rq;
>
> repeat_lock_task:
> rq = task_rq(p);
> spin_lock(&rq->lock);
> if (unlikely(rq != task_rq(p))) {
> spin_unlock(&rq->lock);
> goto repeat_lock_task;
> }
> return rq;
> }
>
> the result of task_rq() depends on p->thread_info->cpu wich will only
> change if a task has migrated over to another CPU. That is a
> fundamentally 'slow' operation, but even if a task does it intentionally
> in a high frequency way (for example via repeated calls to
> sched_setaffinity) there's no way it could be faster than the spinlock
> code here. So ... what problems can you see with it?
OK, you are right - I withdraw this "idea". Sorry!
Jarek P.
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