On Wed, 11 Jul 2007 18:00:58 -0700 "Paul E. McKenney" <[email protected]> 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 <[email protected]>
> ---
>
> 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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