diff options
| author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2007-10-16 23:26:04 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@woody.linux-foundation.org> | 2007-10-17 08:42:46 -0700 |
| commit | 97b430320ce7c95f0d5587c5ecc8f6a9d0c698e9 (patch) | |
| tree | 3f805b4351c2c4ce68b8de869320f2afc9a805d5 | |
| parent | f6b450d489f2fb4e909447beacad64edb8aa0cda (diff) | |
| download | linux-97b430320ce7c95f0d5587c5ecc8f6a9d0c698e9.tar.bz2 | |
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@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: Josh Triplett <josh@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
| -rw-r--r-- | include/linux/rcupdate.h | 14 |
1 files changed, 13 insertions, 1 deletions
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 76c1a530edc5..cc24a01df940 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -231,6 +231,18 @@ extern struct lockdep_map rcu_lock_map; 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 @@ -242,7 +254,7 @@ extern struct lockdep_map rcu_lock_map; */ #define rcu_dereference(p) ({ \ - typeof(p) _________p1 = p; \ + typeof(p) _________p1 = ACCESS_ONCE(p); \ smp_read_barrier_depends(); \ (_________p1); \ }) |