From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932731Ab2LMOjB (ORCPT ); Thu, 13 Dec 2012 09:39:01 -0500 Received: from e7.ny.us.ibm.com ([32.97.182.137]:45841 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932136Ab2LMOi6 (ORCPT ); Thu, 13 Dec 2012 09:38:58 -0500 Date: Thu, 13 Dec 2012 19:38:07 +0530 From: Srikar Dronamraju To: Oleg Nesterov Cc: Ingo Molnar , Peter Zijlstra , Ananth N Mavinakayanahalli , Anton Arapov , linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/7] uprobes: Kill the pointless inode/uc checks in register/unregister Message-ID: <20121213140807.GF29086@linux.vnet.ibm.com> Reply-To: Srikar Dronamraju References: <20121123202741.GA18858@redhat.com> <20121123202806.GA18887@redhat.com> <20121210061901.GF22164@linux.vnet.ibm.com> <20121210191232.GA32462@redhat.com> <20121213103548.GC29086@linux.vnet.ibm.com> <20121213131540.GA11862@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20121213131540.GA11862@redhat.com> User-Agent: Mutt/1.5.20 (2009-06-14) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12121314-5806-0000-0000-00001CEF0D34 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > > > > > > I agree, sometimes it makes sense to protect against the stupid mistakes, > > > but if we want to check against NULL we should do > > > > > > if (WARN_ON(!inode)) > > > return; > > > > > > > agree, that warn_on is better than a simple check > > and this one > > if (WARN_ON(inode < PAGE_OFFSET)) > > is even better ;) Okay. > > > > Especially in uprobe_unregister(). The current code is really "hide > > > the possible problem" and nothing more. It is better to crash imho > > > than silently return. > > > > > > > > register() also checks uc->next == NULL, probably to prevent the > > > > > double-register but the caller can do other stupid/wrong things. > > > > > > > > Users can surely do more stupid things. But this is again something that > > > > kernel can identify. By allowing a double-register of a consumer, thats > > > > already registered, we might end up allowing circular loop of consumers. > > > > > > I understand. But in this case we should document that uc->next must > > > be cleared before uprobe_register(). Or add init_consumer(). > > > > > > And we should change uprobe_unregister() to clear uc->next as well. > > > I think that the code like this > > > > > > uprobe_register(uc); > > > uprobe_unregister(uc); > > > > > > uprobe_register(uc); > > > > > > should work. Currently it doesn't because of this check. > > > > > > > yes, these should work and makes a case to nullify ->next on unregister. > > > > However, what if someone tries > > > > uprobe_register(uc1); > > uprobe_register(uc2); > > uprobe_register(uc1); > > > > i.e somebody tries to re-register uc1, while its active and has a valid > > next. After the re-registration of uc1, the uprobe->consumers will no more reference uc2. > > Yes. And even without uprobe_register(uc2) the result won't be good. > This is like list_add(node). > > > Should we leave this case as a fool shooting himself? > > IMHO yes, or we should create init_consumer() or at least document that > the private ->next member should be nullified. > Okay, Since we agree that its a user mistake. So lets document this and continue with what you propose.