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Date: Wed, 11 Jul 2007 18:00:58 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: linux-kernel@...r.kernel.org Cc: mingo@...e.hu, dipankar@...ibm.com, josht@...ux.vnet.ibm.com, akpm@...ux-foundation.org Subject: [PATCH] Immunize rcu_dereference() against crazy compiler writers 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)) + /** * rcu_dereference - fetch an RCU-protected pointer in an * RCU read-side critical section. This pointer may later @@ -228,7 +240,7 @@ extern int rcu_needs_cpu(int cpu); */ #define rcu_dereference(p) ({ \ - typeof(p) _________p1 = p; \ + typeof(p) _________p1 = ACCESS_ONCE(p); \ smp_read_barrier_depends(); \ (_________p1); \ }) - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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