linux-kernel - Re: PREEMPT_RCU breaks anon

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Date:	Sat, 24 Feb 2007 14:36:55 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Oleg Nesterov <oleg@...sign.ru>, dipankar@...ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: PREEMPT_RCU breaks anon_vma locking ?

On Sat, Feb 24, 2007 at 10:10:57PM +0000, Hugh Dickins wrote:
> On Fri, 23 Feb 2007, Paul E. McKenney wrote:
> > 
> > This look like a valid fix to me, at least as long as the lock is never
> > dropped in the meantime (e.g., to do I/O).  If the lock -is- dropped in
> > the meantime, then presumably whatever is done to keep the page from
> > vanishing should allow an rcu_read_unlock() to be placed after each
> > spin_unlock(&...->lock) and an rcu_read_lock() to be placed before each
> > spin_lock(&...->lock).
> 
> Thankfully no complications of that kind, page_lock_anon_vma is static
> to mm/rmap.c, and only used to hold the spin lock while examining page
> tables of the vmas in the list, never a need to drop that lock at all.
> (Until the day when someone reports such a long list that we start to
> worry about the latency.)

Whew!!!  For now, anyway!  ;-)

						Thanx, Paul
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