On 05/23, Cliff Wickman wrote:
>
> On Thu, May 24, 2007 at 01:29:02AM +0400, Oleg Nesterov wrote:
> > Cliff Wickman wrote:
> > >
> > > - * NOTE: interrupts should be disabled by the caller
> > > + * NOTE: interrupts are not disabled by the caller
> > > */
> > > static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
> > > {
> > > @@ -5008,6 +5008,17 @@ restart:
> > > if (dest_cpu == NR_CPUS)
> > > dest_cpu = any_online_cpu(p->cpus_allowed);
> > >
> > > + /* try to stay on the same cpuset */
> > > + if (dest_cpu == NR_CPUS) {
> > > + /*
> > > + * Call to cpuset_cpus_allowed may sleep, so we depend
> > > + * on move_task_off_dead_cpu() being called in a non-critical
> > > + * region.
> > > + */
> > > + p->cpus_allowed = cpuset_cpus_allowed(p);
> > > + dest_cpu = any_online_cpu(p->cpus_allowed);
> > > + }
> >
> > I know nothing about cpuset.c, a _very_ naive question.
>
> Paul Jackson is the cpuset guru.
Hopefully Paul can help us...
> > Do we really need task_lock() (used by cpuset_cpus_allowed) here ?
>
> According to Paul's comment in kernel/cpuset.c
> * It is ok to first take manage_sem, then nest callback_sem. We also
> * require taking task_lock() when dereferencing a tasks cpuset pointer.
> So I'm afraid it is not safe to call guarantee_online_cpus(tsk->cpuset, &mask);
> without it. Could the task not be exiting?
But how task_lock() can help? cpuset_exit() doesn't take it, it changes ->cpuset
lockless. However, it sets ->cpuset = &top_cpuset, and the comment says:
* Don't even think about derefencing 'cs' after the cpuset use count
* goes to zero, except inside a critical section guarded by manage_mutex
* or callback_mutex.
^^^^^^^^^^^^^^
So, perhaps cpuset_lock() should be enough.
(That said, looking at cpuset_exit() I can't understand why callback_mutex is
enough. If it is not, cpuset_cpus_allowed() is not safe either).
> > If not, probably we can make this simpler. CPU_DEAD takes cpuset_lock(),
> > move_task_off_dead_cpu() uses guarantee_online_cpus() which doesn't sleep,
> > so we don't need other changes.
> >
> > Possible?
> >
> > If not, this patch should also change migrate_dead(), it still calls
> > move_task_off_dead_cpu() with irqs disabled, no?
>
> Right, the lock is released but I indeed didn't reenable irqs.
> How would you suggest doing that? The irq state was saved in local
> variable "flags" back in migration_call().
migration_call(CPU_DEAD) is called with irqs enabled.
Oleg.
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