From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756617AbaLIKap (ORCPT ); Tue, 9 Dec 2014 05:30:45 -0500 Received: from smarthost01a.mail.zen.net.uk ([212.23.1.1]:60081 "EHLO smarthost01a.mail.zen.net.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756116AbaLIKan (ORCPT ); Tue, 9 Dec 2014 05:30:43 -0500 Message-ID: <1418121037.3641.22.camel@linaro.org> Subject: Re: Re: [PATCH v14 7/7] ARM: kprobes: enable OPTPROBES for ARM 32 From: "Jon Medhurst (Tixy)" To: Masami Hiramatsu Cc: Wang Nan , lizefan@huawei.com, linux@arm.linux.org.uk, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Date: Tue, 09 Dec 2014 10:30:37 +0000 In-Reply-To: <5486CB9F.6030804@hitachi.com> References: <1418020040-68977-1-git-send-email-wangnan0@huawei.com> <1418020131-69375-1-git-send-email-wangnan0@huawei.com> <1418036666.3647.33.camel@linaro.org> <5485886E.2060303@huawei.com> <1418039451.3647.48.camel@linaro.org> <5486CB9F.6030804@hitachi.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.9-1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-Originating-smarthost01a-IP: [82.69.122.217] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2014-12-09 at 19:14 +0900, Masami Hiramatsu wrote: > (2014/12/08 20:50), Jon Medhurst (Tixy) wrote:> arch_optimize_kprobes is calling __arch_optimize_kprobes, which is > > iterating over a list of probes and removing each one in turn, if this > > is happening on multiple cpu's simultaneously, it's not clear to me that > > such an operation is safe. list_del_init calls __list_del which does > > > > next->prev = prev; > > prev->next = next; > > > > so what happens if another cpu is at the same time updating any of those > > list entries? Without even fully analysing the code I can see that with > > the fact that the list handling helpers have no memory barriers, that > > the above two lines could be seen to execute in the reverse order, e.g. > > > > prev->next = next; > > next->prev = prev; > > > > so another CPU could find and delete next before this one has finished > > doing so. Would the list end up in a consistent state where no loops > > develop and no probes are missed? I don't know the answer and a full > > analysis would be complicated, but my gut feeling is that if a cpu can > > observe the links in the list in an inconsistent state then only bad > > things can result. > > Just a comment, arch_optimize_kprobes() are only called under > kprobe_mutex held. No concurrent update happens :) Except in the case of the code I was commenting on which was using stop_machine to make all cpu's simultaneously do the work of arch_optimize_kprobes :-) -- Tixy