From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail6.bemta12.messagelabs.com (mail6.bemta12.messagelabs.com [216.82.250.247]) by kanga.kvack.org (Postfix) with ESMTP id 531996B00EE for ; Sun, 24 Jul 2011 14:10:15 -0400 (EDT) Date: Sun, 24 Jul 2011 20:07:13 +0200 From: Oleg Nesterov Subject: Re: [PATCH v4 3.0-rc2-tip 4/22] 4: Uprobes: register/unregister probes. Message-ID: <20110724180713.GA24599@redhat.com> References: <20110607125804.28590.92092.sendpatchset@localhost6.localdomain6> <20110607125900.28590.16071.sendpatchset@localhost6.localdomain6> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110607125900.28590.16071.sendpatchset@localhost6.localdomain6> Sender: owner-linux-mm@kvack.org List-ID: To: Srikar Dronamraju Cc: Peter Zijlstra , 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 , Andrew Morton , Jim Keniston , Roland McGrath , Andi Kleen , LKML Hi Srikar, I still hope some day I'll find the time to read the whole series ;) Trying to continue from where I have stopped, and it seems that this patch has a couple more problems. On 06/07, Srikar Dronamraju wrote: > > A probe is specified by a file:offset. While registering, a breakpoint > is inserted for the first consumer, On subsequent probes, the consumer > gets appended to the existing consumers. While unregistering a > breakpoint is removed if the consumer happens to be the last consumer. > All other unregisterations, the consumer is deleted from the list of > consumers. > > Probe specifications are maintained in a rb tree. A probe specification > is converted into a uprobe before store in a rb tree. A uprobe can be > shared by many consumers. register/unregister logic looks racy... Supose that uprobe U has a single consumer C and register_uprobe() is called with the same inode/offset, while another thread does unregister(U,C). - register() calls alloc_uprobe(), finds the entry in rb tree, and increments U->ref. But this doesn't add the new consumer. - uregister() does del_consumer(), and removes the single consumer C. then it takes uprobes_mutex, sees uprobe->consumers == NULL and calls delete_uprobe()->rb_erase() - register() continues, takes uprobes_mutex, re-inserts the breakpoints, finds the new consumer and succeeds. However, this uprobe is not in rb-tree, it was deleted by unregister. OTOH. Suppose we add the new uprobe. register()->alloc_uprobe() sets new_uprobe->ref == 2. If something goes wrong after that, register() does delete_uprobe() + put_uprobe(), new_uprobe->ref becomes 1 and we leak this uprobe. Oleg. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org