On 06/23, Peter Zijlstra wrote:
>
> On Tue, Jun 23, 2015 at 12:57:39AM +0200, Oleg Nesterov wrote:
> > > +
> > > + lock_map_acquire_read(&cpu_hotplug.rwsem.rw_sem.dep_map);
> > > + _percpu_down_read(&cpu_hotplug.rwsem);
> > > }
> >
> > Confused... Why do we need _percpu_down_read()? Can't get_online_cpus()
> > just use percpu_down_read() ?
> >
> > Yes, percpu_down_read() is not recursive, like the normal down_read().
> > But this does not matter because we rely on ->cpuhp_ref anyway?
>
> While we will not call the actual lock, lockdep will still get confused
> by the inconsistent locking order observed.
>
> Change it and boot, you'll find lockdep output pretty quickly.
Hmm. and I simply can't understand why...
>
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -1410,6 +1410,8 @@ static struct task_struct *copy_process(
> > > p->sequential_io_avg = 0;
> > > #endif
> > >
> > > + cpu_hotplug_init_task(p);
> >
> > This is probably unnecessary, copy_process() should not be called under
> > get_online_cpus().
>
> Probably true, in which case we could still use the callback to insert a
> WARN_ON_ONCE(p->cpuhp_ref) :-)
Yes, agreed.
And, perhaps, WARN_ON_ONCE(in_irq) in try_get_online_cpus() makes sense...
percpu_down_read_trylock() from irq is fine, but try_get_online_cpus()
can come right after get/put_online_cpus() updates ->cpuhp_ref.
And I forgot to say,
> void get_online_cpus(void)
> {
> might_sleep();
> - if (cpu_hotplug.active_writer == current)
> +
> + /* read in write recursion */
> + if (cpu_hotplug.writer == current)
> + return;
...
> void put_online_cpus(void)
> {
> - int refcount;
> -
> - if (cpu_hotplug.active_writer == current)
> + if (cpu_hotplug.writer == current)
> return;
We do not need to check cpu_hotplug.writer in get/put_online_cpus().
cpu_hotplug_begin/end can just inc/dec current->cpuhp_ref.
Oleg.
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