From 88c1863066ccfa456797e12c5d8b4631aa1ad0d0 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Mon, 16 Dec 2013 13:24:32 -0800 Subject: [PATCH] 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 Reviewed-by: Josh Triplett --- include/linux/rcupdate.h | 8 ++------ 1 file changed, 2 insertions(+), 6 deletions(-) diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 278a9da69ec4..32decf1a9c6c 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -44,6 +44,7 @@ #include #include #include +#include #ifdef CONFIG_RCU_TORTURE_TEST extern int rcutorture_runnable; /* for sysctl */ @@ -580,12 +581,7 @@ static inline void rcu_preempt_sleep_check(void) * 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 -- 2.34.1