Date: Thu, 29 Nov 2007 06:36:50 -0800 From: "Paul E. McKenney" Subject: Re: [PATCH (resubmit)][BRIDGE] Properly dereference the br_should_route_hook Message-ID: <20071129143650.GD32449@linux.vnet.ibm.com> References: <474C43F4.5080704@openvz.org> <20071129130420.GA8487@gondor.apana.org.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20071129130420.GA8487@gondor.apana.org.au> Reply-To: paulmck@linux.vnet.ibm.com List-Id: Linux Ethernet Bridging List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Herbert Xu Cc: Linux Netdev List , bridge@lists.osdl.org, devel@openvz.org, Pavel Emelyanov On Fri, Nov 30, 2007 at 12:04:20AM +1100, Herbert Xu wrote: > On Tue, Nov 27, 2007 at 07:21:08PM +0300, Pavel Emelyanov wrote: > > This hook is protected with the RCU, so simple > > > > if (br_should_route_hook) > > br_should_route_hook(...) > > > > is not enough on some architectures. > > > > Use the rcu_dereference/rcu_assign_pointer in this case. > > > > Fixed Stephen's comment concerning using the typeof(). > > > > Signed-off-by: Pavel Emelyanov > > Applied to net-2.6. Thanks Pavel! > > > static void __exit ebtable_broute_fini(void) > > { > > - br_should_route_hook = NULL; > > + rcu_assign_pointer(br_should_route_hook, NULL); > > Just for the record, rcu_assign_pointer is never necessary when > you're assigning NULL. The reason is that rcu_assign_pointer serves > as a barrier between the initialisation of the content of what you're > assigning and the actual assignment. Since NULL does not need to be > initialised you don't need the barrier :) Of course, if the rcu_assign_pointer() of NULL is not on a hot code path, the extra memory barrier might not be hurting enough to care. > Hmm, perhaps we could even build this logic into rcu_assign_pointer. That certainly is an interesting tradeoff... Save a memory barrier when assigning NULL, but pay an extra test and branch in all cases. Though it does make for a simpler rule -- just use rcu_assign_pointer() in all cases. Of course, if almost all rcu_assign_pointer() executions assign non-NULL pointers, the optimal strategy would be to leave the implementation of rcu_assign_pointer() alone, and simply enforce use of rcu_assign_pointer(), even if the pointer being assigned is NULL. For a rough guess, if fewer than a few percent of rcu_assign_pointer() executions assign NULL, then it is best to simply change the rule. If more than about ten percent of rcu_assign_pointer() executions assign NULL, then it would make sense to put the check into the rcu_assign_pointer() primitive. The percentages would be of dynamic executions, rather than static counts of lines of code. So, any intuitions on what fraction of the time rcu_assign_pointer() is assigning NULL? Failing that, what workload should be used to take the measurements? ;-) > Then again, who still uses an Alpha? Mine died years ago :) Although rcu_dereference() does a memory barrier only on Alpha, that of rcu_assign_pointer() is needed on any machine that does not preserve store order (Itanium, POWER, ARM, some MIPS boxes according to rumor, ...). Thanx, Paul > Cheers, > -- > Visit Openswan at http://www.openswan.org/ > Email: Herbert Xu ~{PmV>HI~} > Home Page: http://gondor.apana.org.au/~herbert/ > PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt > - > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html