linux-kernel - Re: [PATCH] Immunize rcu_dereference() against crazy compiler writers

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Message-Id: <20070717024600.0aab7f36.akpm@linux-foundation.org>
Date:	Tue, 17 Jul 2007 02:46:00 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
	josht@...ux.vnet.ibm.com
Subject: Re: [PATCH] Immunize rcu_dereference() against crazy compiler
 writers

On Wed, 11 Jul 2007 18:00:58 -0700 "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> Turns out that compiler writers are a bit more aggressive about optimizing
> than one might expect.  This patch prevents a number of such optimizations
> from messing up rcu_deference().  This is not merely a theoretical
> problem, as evidenced by the rmb() in mce_log().
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
> 
>  rcupdate.h |   14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff -urpNa -X dontdiff linux-2.6.22/include/linux/rcupdate.h linux-2.6.22-volrcud/include/linux/rcupdate.h
> --- linux-2.6.22/include/linux/rcupdate.h	2007-07-08 16:32:17.000000000 -0700
> +++ linux-2.6.22-volrcud/include/linux/rcupdate.h	2007-07-11 17:21:09.000000000 -0700
> @@ -217,6 +217,18 @@ extern int rcu_needs_cpu(int cpu);
>  		local_bh_enable(); \
>  	} while(0)
>  
> +/*
> + * Prevent the compiler from merging or refetching accesses.  The compiler
> + * is also forbidden from reordering successive instances of ACCESS_ONCE(),
> + * but only when the compiler is aware of some particular ordering.  One way
> + * to make the compiler aware of ordering is to put the two invocations of 
> + * ACCESS_ONCE() in different C statements.
> + *
> + * This macro does absolutely -nothing- to prevent the CPU from reordering,
> + * merging, or refetching absolutely anything at any time.
> + */
> +#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))

drivers/net/hamradio/bpqether.c: In function 'bpq_seq_next':
drivers/net/hamradio/bpqether.c:421: error: invalid lvalue in unary '&'
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