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

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jun 24, 2015 at 03:50:49PM +0200, Oleg Nesterov wrote:
> On 06/23, Peter Zijlstra wrote:
> > 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.

I'm confused.. why isn't the read-in-read recursion good enough?