From mboxrd@z Thu Jan 1 00:00:00 1970 From: Pablo Neira Ayuso Subject: Re: [PATCH nf] netfilter: invoke synchronize_rcu after set the _hook_ to NULL Date: Fri, 24 Mar 2017 19:45:16 +0100 Message-ID: <20170324184516.GA2388@salvia> References: <1490015030-16403-1-git-send-email-zlpnobody@163.com> <20170324121707.GA2764@salvia> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Liping Zhang , Netfilter Developer Mailing List To: Liping Zhang Return-path: Received: from mail.us.es ([193.147.175.20]:53792 "EHLO mail.us.es" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751410AbdCXSpX (ORCPT ); Fri, 24 Mar 2017 14:45:23 -0400 Received: from antivirus1-rhel7.int (unknown [192.168.2.11]) by mail.us.es (Postfix) with ESMTP id B2D6C1EC2C4 for ; Fri, 24 Mar 2017 19:45:19 +0100 (CET) Received: from antivirus1-rhel7.int (localhost [127.0.0.1]) by antivirus1-rhel7.int (Postfix) with ESMTP id A570BFC2DC for ; Fri, 24 Mar 2017 19:45:19 +0100 (CET) Received: from antivirus1-rhel7.int (localhost [127.0.0.1]) by antivirus1-rhel7.int (Postfix) with ESMTP id 4453EFB471 for ; Fri, 24 Mar 2017 19:45:16 +0100 (CET) Content-Disposition: inline In-Reply-To: Sender: netfilter-devel-owner@vger.kernel.org List-ID: On Fri, Mar 24, 2017 at 09:01:17PM +0800, Liping Zhang wrote: > Hi Pablo, > > 2017-03-24 20:17 GMT+08:00 Pablo Neira Ayuso : > >> --- a/net/netfilter/nfnetlink_cttimeout.c > >> +++ b/net/netfilter/nfnetlink_cttimeout.c > >> @@ -646,8 +646,8 @@ static void __exit cttimeout_exit(void) > >> #ifdef CONFIG_NF_CONNTRACK_TIMEOUT > >> RCU_INIT_POINTER(nf_ct_timeout_find_get_hook, NULL); > >> RCU_INIT_POINTER(nf_ct_timeout_put_hook, NULL); > >> + synchronize_rcu(); > >> #endif /* CONFIG_NF_CONNTRACK_TIMEOUT */ > >> - rcu_barrier(); > > > > cttimeout relies on kfree_rcu(). > > > > Are you sure we don't need this? > > > > According to: > > > > https://www.kernel.org/doc/Documentation/RCU/rcubarrier.txt > > > > "We could try placing a synchronize_rcu() in the module-exit code path, > > but this is not sufficient." > > > > Then: > > > > "Please note that rcu_barrier() does -not- imply synchronize_rcu(), in > > particular, if there are no RCU callbacks queued anywhere, > > rcu_barrier() is within its rights to return immediately, without > > waiting for a grace period to elapse." > > This is because we use kfree_rcu to free the cttimeout objects. So I think > rcu_barrier() is not needed anymore. > > Quoted from https://lwn.net/Articles/433493/ : > "And kfree_rcu() is also help for unloadable modules, kfree_rcu() does not > queue any function which belong to the module, so a rcu_barrier() can > be avoid when module exit." > > Also from commit 9ab1544eb419 ("rcu: introduce kfree_rcu()"): > "Many rcu callbacks functions just call kfree() on the base structure. > These functions are trivial, but their size adds up, and furthermore > when they are used in a kernel module, that module must invoke the > high-latency rcu_barrier() function at module-unload time." Right, thanks for explaining. I think we can get this smaller: it should be possible to avoid this synchronize_rcu() call from nf_conntrack_{register,unregister}_notifier(). These two are called from ctnetlink netns path, this patch already adds a synchronize_rcu() spot to ctnetlink module removal which is where the event callback can vanish. You can just add a comment there so we don't forget about this, eg. @@ -290,6 +290,7 @@ void nf_conntrack_unregister_notifier(struct net *net, BUG_ON(notify != new); RCU_INIT_POINTER(net->ct.nf_conntrack_event_cb, NULL); mutex_unlock(&nf_ct_ecache_mutex); + /* synchronize_rcu() is called from ctnetlink. */ } What do you think?