Re: [RFC][PATCH 09/13] hotplug: Replace hotplug lock with percpu-rwsem

[Oleg Nesterov <oleg@redhat.com>]

On 06/23, Peter Zijlstra wrote:
>
> On Tue, Jun 23, 2015 at 07:01:22PM +0200, Oleg Nesterov wrote:
> > 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...
>
> If in one callchain we do:
>
> 	get_online_cpus();
> 	lock(A);
>
> in another we do:
>
> 	lock(A);
> 	get_online_cpus();
>
> lockdep will complain about the inverted lock order, however this is not
> a problem at all for recursive locks.

Ah, but in this case lockdep is right. This is deadlockable because
with the new implementation percpu_down_write() blocks the new readers.
So this change just hides the valid warning.

Just suppose that the 3rd CPU does percpu_down_write()->down_write()
right after the 2nd CPU (above) takes lock(A).

I have to admit that I didn't realize that the code above is currently
correct... but it is.

So we need percpu_down_write_dont_block_readers(). I already thought
about this before, I'll try to make the patch tomorrow on top of your
changes.

This means that we do not need task_struct->cpuhp_ref, but we can't
avoid livelock we currently have: cpu_hotplug_begin() can never succeed
if the new readers come fast enough.

Oleg.