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Subject: Re: [PATCH 00/21] mm: Preemptibility -v6
From: Peter Zijlstra <peterz@infradead.org>
To: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
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        Oleg Nesterov <oleg@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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Date: Mon, 24 Jan 2011 13:21:54 +0100
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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.


* possibly ARM and SH don't either, but I can't read either ASMs well
enough to tell.