linux-kernel - Re: [PATCH] adapt page_lock_anon_vma() to PREEMPT

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Date:	Tue, 27 Feb 2007 15:13:15 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...sign.ru>, hugh@...itas.com,
	clameter@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] adapt page_lock_anon_vma() to PREEMPT_RCU

On Tue, Feb 27, 2007 at 12:25:17PM -0800, Andrew Morton wrote:
> > On Sun, 25 Feb 2007 23:06:21 +0300 Oleg Nesterov <oleg@...sign.ru> wrote:
> > page_lock_anon_vma() uses spin_lock() to block RCU. This doesn't work with
> > PREEMPT_RCU, we have to do rcu_read_lock() explicitely. Otherwise, it is
> > theoretically possible that slab returns anon_vma's memory to the system
> > before we do spin_unlock(&anon_vma->lock).
> > 
> > ...
> >
> > +static void page_unlock_anon_vma(struct anon_vma *anon_vma)
> > +{
> > +	spin_unlock(&anon_vma->lock);
> > +	rcu_read_unlock();
> >  }
> 
> It's a bit sad doing a double preempt_disable() for non-PREEMPT_RCU builds.
> 
> Perhaps we would benefit from a new rcu_read_lock_preempt_rcu() which is a
> no-op if !PREEMPT_RCU.

We were doing double preempt_disable() before as well.  The only
difference is that we moved RCU preempt_enable() (it used to be inside the
critical section, and now it is after the corresponding spin_unlock()).

I hope to keep RCU API proliferation down to a dull roar, but if we
need more APIs, we need more APIs.  This example does not demonstrate
that need to me, however.

						Thanx, Paul
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