From: "Paul E. McKenney" 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 Cc: Ingo Molnar Acked-by: Josh Triplett Signed-off-by: Andrew Morton --- include/linux/rcupdate.h | 14 +++++++++++++- 1 files changed, 13 insertions(+), 1 deletion(-) diff -puN include/linux/rcupdate.h~immunize-rcu_dereference-against-crazy-compiler-writers include/linux/rcupdate.h --- a/include/linux/rcupdate.h~immunize-rcu_dereference-against-crazy-compiler-writers +++ a/include/linux/rcupdate.h @@ -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); \ }) _