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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2007-10-17 10:26:04 +0400
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-10-17 19:42:46 +0400
commit97b430320ce7c95f0d5587c5ecc8f6a9d0c698e9 (patch)
tree3f805b4351c2c4ce68b8de869320f2afc9a805d5
parentf6b450d489f2fb4e909447beacad64edb8aa0cda (diff)
downloadlinux-97b430320ce7c95f0d5587c5ecc8f6a9d0c698e9.tar.xz
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.h14
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); \
})