linux-kernel - Re: [PATCH 00/21] mm: Preemptibility -v6

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Date:	Mon, 24 Jan 2011 16:00:46 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Nick Piggin <npiggin@...nel.dk>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org, Andrea Arcangeli <aarcange@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 00/21] mm: Preemptibility -v6

On Mon, 2011-01-24 at 15:34 +0100, Oleg Nesterov wrote:
> On 01/24, Peter Zijlstra wrote:
> >
> > On Fri, 2011-01-21 at 16:33 +0100, Peter Zijlstra wrote:
> >
> > > Index: linux-2.6/mm/rmap.c
> > > ===================================================================
> > > --- linux-2.6.orig/mm/rmap.c
> > > +++ linux-2.6/mm/rmap.c
> > > @@ -1559,9 +1559,20 @@ void __put_anon_vma(struct anon_vma *ano
> > >  	 * Synchronize against page_lock_anon_vma() such that
> > >  	 * we can safely hold the lock without the anon_vma getting
> > >  	 * freed.
> > > +	 *
> > > +	 * Relies on the full mb implied by the atomic_dec_and_test() from
> > > +	 * put_anon_vma() against the full mb implied by mutex_trylock() from
> > > +	 * page_lock_anon_vma(). This orders:
> > > +	 *
> > > +	 * page_lock_anon_vma()		VS	put_anon_vma()
> > > +	 *   mutex_trylock()			  atomic_dec_and_test()
> > > +	 *   smp_mb()				  smp_mb()
> > > +	 *   atomic_read()			  mutex_is_locked()
> >
> > Bah!, I thought all mutex_trylock() implementations used an atomic op
> > with return value (which implies a mb), but it looks like (at least*)
> > PPC doesn't and only provides a LOCK barrier.
> 
> But, mutex_trylock() must imply the one-way barrier, otherwise it
> is buggy, no?

> If this atomic_read() can leak out of the critical section, then
> I think mutex_trylock() should be fixed. Or I misunderstood the
> problem completely...

It implies the LOCK barrier, the one way permeable thing, not a full mb.

But I'm not sure the LOCK is sufficient to make the above scenario work.

> BTW, from https://lkml.org/lkml/2010/11/26/213
> 
> 	+ * Similar to page_get_anon_vma() except it locks the anon_vma.
> 	...
> 	-	struct anon_vma *anon_vma = page_get_anon_vma(page);
> 
> looks like, page_get_anon_vma() becomes unused.

In my current tree there's two left in mm/migrate.c.


git://git.kernel.org/pub/scm/linux/kernel/git/peterz/linux-2.6-mmu_preempt.git

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