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author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2013-12-17 01:24:32 +0400 |
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committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2014-02-18 03:02:00 +0400 |
commit | 88c1863066ccfa456797e12c5d8b4631aa1ad0d0 (patch) | |
tree | 4f456425e3af718957113bd9c4b019b10c496061 /include/linux/rcupdate.h | |
parent | 0adab9b9aa18d7e90337d43567f1eec3d5401b81 (diff) | |
download | linux-88c1863066ccfa456797e12c5d8b4631aa1ad0d0.tar.xz |
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>
Diffstat (limited to 'include/linux/rcupdate.h')
-rw-r--r-- | include/linux/rcupdate.h | 8 |
1 files 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 <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 @@ 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 |