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

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Date:	Tue, 23 Jun 2015 19:01:22 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	paulmck@...ux.vnet.ibm.com, tj@...nel.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org, der.herr@...r.at, dave@...olabs.net,
	riel@...hat.com, viro@...IV.linux.org.uk,
	torvalds@...ux-foundation.org
Subject: Re: [RFC][PATCH 09/13] hotplug: Replace hotplug lock with
	percpu-rwsem

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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