From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Mon, 16 Dec 2013 21:24:32 +0000 (-0800)
Subject: rcu: Define rcu_assign_pointer() in terms of smp_store_release()
X-Git-Tag: v3.15-rc1~183^2~1^2~1^2~6
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=88c1863066ccfa456797e12c5d8b4631aa1ad0d0;p=pandora-kernel.git

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>
---

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