Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752235Ab1FPFem (ORCPT ); Thu, 16 Jun 2011 01:34:42 -0400 Received: from e35.co.us.ibm.com ([32.97.110.153]:40232 "EHLO e35.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751040Ab1FPFel (ORCPT ); Thu, 16 Jun 2011 01:34:41 -0400 Date: Thu, 16 Jun 2011 10:56:39 +0530 From: Srikar Dronamraju To: Peter Zijlstra Cc: Ingo Molnar , Steven Rostedt , Linux-mm , Arnaldo Carvalho de Melo , Linus Torvalds , Ananth N Mavinakayanahalli , Hugh Dickins , Christoph Hellwig , Jonathan Corbet , Thomas Gleixner , Masami Hiramatsu , Oleg Nesterov , Andrew Morton , Jim Keniston , Roland McGrath , Andi Kleen , LKML Subject: Re: [PATCH v4 3.0-rc2-tip 4/22] 4: Uprobes: register/unregister probes. Message-ID: <20110616052639.GI4952@linux.vnet.ibm.com> Reply-To: Srikar Dronamraju References: <20110607125804.28590.92092.sendpatchset@localhost6.localdomain6> <20110607125900.28590.16071.sendpatchset@localhost6.localdomain6> <1307660606.2497.1770.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <1307660606.2497.1770.camel@laptop> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2745 Lines: 63 * Peter Zijlstra [2011-06-10 01:03:26]: > On Tue, 2011-06-07 at 18:29 +0530, Srikar Dronamraju wrote: > > +/* > > + * There could be threads that have hit the breakpoint and are entering the > > + * notifier code and trying to acquire the uprobes_treelock. The thread > > + * calling delete_uprobe() that is removing the uprobe from the rb_tree can > > + * race with these threads and might acquire the uprobes_treelock compared > > + * to some of the breakpoint hit threads. In such a case, the breakpoint hit > > + * threads will not find the uprobe. Finding if a "trap" instruction was > > + * present at the interrupting address is racy. Hence provide some extra > > + * time (by way of synchronize_sched() for breakpoint hit threads to acquire > > + * the uprobes_treelock before the uprobe is removed from the rbtree. > > + */ > > 'some' extra time doesn't really sound convincing to me. Either it is > sufficient to avoid the race or it is not. It reads to me like: we add a > delay so that the race mostly doesn't occur. Not good ;-) The extra time provided is sufficient to avoid the race. So will modify it to mean "sufficient" instead of "some". > > > +static void delete_uprobe(struct uprobe *uprobe) > > +{ > > + unsigned long flags; > > + > > + synchronize_sched(); > > + spin_lock_irqsave(&uprobes_treelock, flags); > > + rb_erase(&uprobe->rb_node, &uprobes_tree); > > + spin_unlock_irqrestore(&uprobes_treelock, flags); > > + iput(uprobe->inode); > > +} > > Also what are the uprobe lifetime rules here? Does it still exist after > this returns? > > The comment in del_consumer() that says: 'drop creation ref' worries me > and makes me thing that is the last reference around and the uprobe will > be freed right there, which clearly cannot happen since its not yet > removed from the RB-tree. > When del_consumer() is called in unregister_uprobe() it has atleast two (or more if the uprobe is hit) references. One at the creation time and the other thro find_uprobe() called in unregister_uprobe before del_consumer. So the reference lost in del_consumer is never the last reference. I added a commented this as creation reference so that the find_uprobe and the put_uprobe() before return would match. If the comment is confusing I can delete it or reword it as suggested by Steven Rostedt which is /* Have caller drop the creation ref */ I would prefer to delete the comment. -- Thanks and Regards Srikar -- 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/