From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: [PATCH 1/2] remove rcu_assign_pointer(NULL) penalty with type/macro safety Date: Wed, 13 Feb 2008 14:00:24 -0800 Message-ID: <20080213220024.GA30729@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: shemminger@vyatta.com, davem@davemloft.net, netdev@vger.kernel.org, dipankar@in.ibm.com, ego@in.ibm.com, herbert@gondor.apana.org.au, akpm@linux-foundation.org To: linux-kernel@vger.kernel.org Return-path: Received: from e6.ny.us.ibm.com ([32.97.182.146]:46107 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763068AbYBMWBd (ORCPT ); Wed, 13 Feb 2008 17:01:33 -0500 Content-Disposition: inline Sender: netdev-owner@vger.kernel.org List-ID: Hello! This is an updated version of the patch posted last November: http://archives.free.net.ph/message/20071201.003721.cd6ff17c.en.html This new version permits arguments with side effects, for example: rcu_assign_pointer(global_p, p++); and also verifies that the arguments are pointers, while still avoiding the unnecessary memory barrier when assigning NULL to a pointer. This memory-barrier avoidance means that rcu_assign_pointer() is now only permitted for pointers (not array indexes), and so this version emits a compiler warning if the first argument is not a pointer. I built a "make allyesconfig" version on an x86 system, and received no such warnings. If RCU is ever applied to array indexes, then the second patch in this series should be applied, and the resulting rcu_assign_index() be used. Given the rather surprising history of subtlely broken implementations of rcu_assign_pointer(), I took the precaution of generating a full set of test cases and verified that memory barriers and compiler warnings were emitted when required. I guess it is the simple things that get you... Signed-off-by: Paul E. McKenney --- rcupdate.h | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) diff -urpNa -X dontdiff linux-2.6.24/include/linux/rcupdate.h linux-2.6.24-rap/include/linux/rcupdate.h --- linux-2.6.24/include/linux/rcupdate.h 2008-01-24 14:58:37.000000000 -0800 +++ linux-2.6.24-rap/include/linux/rcupdate.h 2008-02-13 13:36:47.000000000 -0800 @@ -270,12 +270,20 @@ extern struct lockdep_map rcu_lock_map; * structure after the pointer assignment. More importantly, this * call documents which pointers will be dereferenced by RCU read-side * code. + * + * Throws a compiler warning for non-pointer arguments. + * + * Does not insert a memory barrier for a NULL pointer. */ -#define rcu_assign_pointer(p, v) ({ \ - smp_wmb(); \ - (p) = (v); \ - }) +#define rcu_assign_pointer(p, v) \ + ({ \ + typeof(*p) *_________p1 = (v); \ + \ + if (!__builtin_constant_p(v) || (_________p1 != NULL)) \ + smp_wmb(); \ + (p) = _________p1; \ + }) /** * synchronize_sched - block until all CPUs have exited any non-preemptive