Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753989AbbFQAdd (ORCPT ); Tue, 16 Jun 2015 20:33:33 -0400 Received: from mail-pa0-f53.google.com ([209.85.220.53]:35570 "EHLO mail-pa0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752356AbbFQAd0 (ORCPT ); Tue, 16 Jun 2015 20:33:26 -0400 Message-ID: <5580C054.2080809@plumgrid.com> Date: Tue, 16 Jun 2015 17:33:24 -0700 From: Alexei Starovoitov User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Steven Rostedt CC: Daniel Wagner , paulmck@linux.vnet.ibm.com, Daniel Wagner , LKML Subject: Re: call_rcu from trace_preempt References: <557F509D.2000509@plumgrid.com> <20150615230702.GB3913@linux.vnet.ibm.com> <557F7764.5060707@plumgrid.com> <20150616021458.GE3913@linux.vnet.ibm.com> <557FB7E1.6080004@plumgrid.com> <20150616122733.GG3913@linux.vnet.ibm.com> <558018DD.1080701@monom.org> <55805AC5.8020507@plumgrid.com> <20150616133709.6c53645d@gandalf.local.home> In-Reply-To: <20150616133709.6c53645d@gandalf.local.home> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1327 Lines: 26 On 6/16/15 10:37 AM, Steven Rostedt wrote: >>> + kfree(l); >> > >> >that's not right, since such thread defeats rcu protection of lookup. >> >We need either kfree_rcu/call_rcu or synchronize_rcu. >> >Obviously the former is preferred that's why I'm still digging into it. >> >Probably a thread that does kfree_rcu would be ok, but we shouldn't >> >be doing it unconditionally. For all networking programs and 99% >> >of tracing programs the existing code is fine and I don't want to >> >slow it down to tackle the corner case. >> >Extra spin_lock just to add it to the list is also quite costly. > Use a irq_work() handler to do the kfree_rcu(), and use llist (lockless > list) to add items to the list. have been studying irq_work and llist... it will work, but it's quite costly too. Every kfree_rcu will be replaced with irq_work_queue(), which is irq_work_claim() with one lock_cmpxchg plus another lock_cmpxchg in llist_add, plus another lock_cmpxchg for our own llist of 'to be kfree_rcu-ed htab elements'. That's a lot. The must be better solution. Need to explore more. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/