rcu: Define rcu_assign_pointer() in terms of smp_store_release()
The new smp_store_release() function provides better guarantees than did
rcu_assign_pointer(), and potentially less overhead on some architectures.
The guarantee that smp_store_release() provides that rcu_assign_pointer()
does that is obscure, but its lack could cause considerable confusion.
This guarantee is illustrated by the following code fragment:
struct foo {
int a;
int b;
int c;
struct foo *next;
};
struct foo foo1;
struct foo foo2;
struct foo __rcu *foop;
...
foo2.a = 1;
foo2.b = 2;
BUG_ON(foo2.c);
rcu_assign_pointer(foop, &foo);
...
fp = rcu_dereference(foop);
fp.c = 3;
The current rcu_assign_pointer() semantics permit the BUG_ON() to
trigger because rcu_assign_pointer()'s smp_wmb() is not guaranteed to
order prior reads against later writes. This commit therefore upgrades
rcu_assign_pointer() from smp_wmb() to smp_store_release() to avoid this
counter-intuitive outcome.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 278a9da..32decf1 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -44,6 +44,7 @@
#include <linux/debugobjects.h>
#include <linux/bug.h>
#include <linux/compiler.h>
+#include <asm/barrier.h>
#ifdef CONFIG_RCU_TORTURE_TEST
extern int rcutorture_runnable; /* for sysctl */
@@ -580,12 +581,7 @@
* please be careful when making changes to rcu_assign_pointer() and the
* other macros that it invokes.
*/
-#define rcu_assign_pointer(p, v) \
- do { \
- smp_wmb(); \
- ACCESS_ONCE(p) = RCU_INITIALIZER(v); \
- } while (0)
-
+#define rcu_assign_pointer(p, v) smp_store_release(&p, RCU_INITIALIZER(v))
/**
* rcu_access_pointer() - fetch RCU pointer with no dereferencing