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Date: Mon, 24 Jan 2011 15:34:07 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Hugh Dickins <hughd@google.com>, Andrew Morton <akpm@linux-foundation.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        David Miller <davem@davemloft.net>, Nick Piggin <npiggin@kernel.dk>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-mm@kvack.org, Andrea Arcangeli <aarcange@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [PATCH 00/21] mm: Preemptibility -v6
Message-ID: <20110124143407.GA19856@redhat.com>
References: <20101126143843.801484792@chello.nl> <alpine.LSU.2.00.1101172301340.2899@sister.anvils> <1295457039.28776.137.camel@laptop> <alpine.LSU.2.00.1101201052060.1603@sister.anvils> <1295624034.28776.303.camel@laptop> <1295871714.28776.406.camel@laptop>
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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?

IOW, page_lock_anon_vma() does:

	if (mutex_trylock(anon_vma->root->lock)) {
		...
		atomic_read(&anon_vma->refcount);
		...
	}

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...

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.

Oleg.