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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>
This commit is contained in:
Paul E. McKenney 2013-12-16 13:24:32 -08:00
parent 0adab9b9aa
commit 88c1863066
1 changed files with 2 additions and 6 deletions
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8
include/linux/rcupdate.h
View File

@ -44,6 +44,7 @@
#include <linux/debugobjects.h> #include <linux/debugobjects.h>
#include <linux/bug.h> #include <linux/bug.h>
#include <linux/compiler.h> #include <linux/compiler.h>
#include <asm/barrier.h>
#ifdef CONFIG_RCU_TORTURE_TEST #ifdef CONFIG_RCU_TORTURE_TEST
extern int rcutorture_runnable; /* for sysctl */ 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 * please be careful when making changes to rcu_assign_pointer() and the
* other macros that it invokes. * other macros that it invokes.
*/ */
#define rcu_assign_pointer(p, v) \ #define rcu_assign_pointer(p, v) smp_store_release(&p, RCU_INITIALIZER(v))
do { \
smp_wmb(); \
ACCESS_ONCE(p) = RCU_INITIALIZER(v); \
} while (0)
/** /**
* rcu_access_pointer() - fetch RCU pointer with no dereferencing * rcu_access_pointer() - fetch RCU pointer with no dereferencing