Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754241Ab3J3UAw (ORCPT ); Wed, 30 Oct 2013 16:00:52 -0400 Received: from caramon.arm.linux.org.uk ([78.32.30.218]:42044 "EHLO caramon.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752453Ab3J3UAv (ORCPT ); Wed, 30 Oct 2013 16:00:51 -0400 Date: Wed, 30 Oct 2013 20:00:31 +0000 From: Russell King - ARM Linux To: Linus Torvalds Cc: linux-mm , linux-fsdevel , Linux Kernel Mailing List , Dave Chinner , Al Viro , Andrew Morton Subject: Re: [PATCH] mm: list_lru: fix almost infinite loop causing effective livelock Message-ID: <20131030200030.GO16735@n2100.arm.linux.org.uk> References: <20131030141616.GB16735@n2100.arm.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2097 Lines: 53 On Wed, Oct 30, 2013 at 12:49:05PM -0700, Linus Torvalds wrote: > On Wed, Oct 30, 2013 at 7:16 AM, Russell King - ARM Linux > wrote: > > > > So, if *nr_to_walk was zero when this function was entered, that means > > we're wanting to operate on (~0UL)+1 objects - which might as well be > > infinite. > > > > Clearly this is not correct behaviour. If we think about the behaviour > > of this function when *nr_to_walk is 1, then clearly it's wrong - we > > decrement first and then test for zero - which results in us doing > > nothing at all. A post-decrement would give the desired behaviour - > > we'd try to walk one object and one object only if *nr_to_walk were > > one. > > > > It also gives the correct behaviour for zero - we exit at this point. > > Good analysis. > > HOWEVER. > > I actually think even your version is very dangerous, because we pass > in the *address* to that count, and the only real reason to do that is > because we might call it in a loop, and we want the function to update > that count. > > And even your version still underflows from 0 to really-large-count. > It *returns* when underflow happens, but you end up with the counter > updated to a large value, and then anybody who uses it later would be > screwed. Yes, you're right... my failing case thankfully doesn't make use of the counter again which is probably why I didn't think about that aspect. > So I think we should either change that "unsigned long" to just > "long", and then check for "<= 0" (like list_lru_walk() already does), > or we should do > > if (!*nr_to_walk) > break; > --*nr_to_walk; > > to make sure that we never do that underflow. > > I will modify your patch to do the latter, since it's the smaller > change, but I suspect we should think about making that thing signed. Thanks... :) -- 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/