Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754530Ab3J3TtI (ORCPT ); Wed, 30 Oct 2013 15:49:08 -0400 Received: from mail-vb0-f50.google.com ([209.85.212.50]:48564 "EHLO mail-vb0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754187Ab3J3TtG (ORCPT ); Wed, 30 Oct 2013 15:49:06 -0400 MIME-Version: 1.0 In-Reply-To: <20131030141616.GB16735@n2100.arm.linux.org.uk> References: <20131030141616.GB16735@n2100.arm.linux.org.uk> Date: Wed, 30 Oct 2013 12:49:05 -0700 X-Google-Sender-Auth: c7xz9Eo9CphQNatafasSW5vHLFA Message-ID: Subject: Re: [PATCH] mm: list_lru: fix almost infinite loop causing effective livelock From: Linus Torvalds To: Russell King - ARM Linux Cc: linux-mm , linux-fsdevel , Linux Kernel Mailing List , Dave Chinner , Al Viro , Andrew Morton Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1903 Lines: 53 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. See, for example, the inline list_lru_walk() function in 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. Hmm? Linus -- 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/