On 12/10, David Hildenbrand wrote:
>
> > Why active_writer should see .puts_pending != 0 if this is called
> > right after cpu_hotplug_begin() takes cpu_hotplug.lock but before
> > it sets TASK_UNINTERRUPTIBLE?
>
> get_online_cpus() increased the refcount.
> put_online_cpus() will increment puts_pending and trigger a wake up (if the
> lock is alread taken - might be by cpu_hotplug_begin() or by some other
> get_online_cpus()).
>
> So refcount == 1, puts_pending == 1
>
> cpu_hotplug_begin() gets the lock and sees refcount == 1 and puts_pending == 0
> or puts_pending == 1 (race with put_online_cpus()).
>
> If that answers your question :)
Sorry for confusion ;)
I meant that without mb() cpu_hotplug_begin() can miss puts_pending != 0,
so it needs set_current_state() before atomic_read().
But this doesn't matter, your v4 uses wake_up/prepare_to_wait.
> > IOW,
> >
> > > void cpu_hotplug_begin(void)
> > > {
> > > + spin_lock(&cpu_hotplug.awr_lock);
> > > cpu_hotplug.active_writer = current;
> > > + spin_unlock(&cpu_hotplug.awr_lock);
> > >
> > > cpuhp_lock_acquire();
> > > for (;;) {
> > > mutex_lock(&cpu_hotplug.lock);
> > > + __set_current_state(TASK_UNINTERRUPTIBLE);
> >
> > don't we need set_current_state() here ?
>
> Hm, good question, this was only a move of existing code. But I thing the
> checked variant would be better.
>
> >
> > Oleg.
> >
>
> Thanks!
>
> David
>
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