Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754735AbbBHEZo (ORCPT ); Sat, 7 Feb 2015 23:25:44 -0500 Received: from mail.efficios.com ([78.47.125.74]:47557 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752490AbbBHEZm (ORCPT ); Sat, 7 Feb 2015 23:25:42 -0500 Date: Sun, 8 Feb 2015 04:25:46 +0000 (UTC) From: Mathieu Desnoyers To: Michael Cree Cc: Greg KH , linux-alpha@vger.kernel.org, Richard Henderson , Ivan Kokshaysky , Matt Turner , Huang Ying , linux-kernel@vger.kernel.org, Paul McKenney , David Howells , Pranith Kumar , stable@vger.kernel.org Message-ID: <1374616625.90941.1423369546750.JavaMail.zimbra@efficios.com> In-Reply-To: <20150208004729.GA10472@omega> References: <1423274934-10625-1-git-send-email-mathieu.desnoyers@efficios.com> <20150207221625.GB31119@kroah.com> <1117480383.90061.1423348244225.JavaMail.zimbra@efficios.com> <20150208004729.GA10472@omega> Subject: Re: [PATCH] llist: Fix missing lockless_dereference() MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [192.222.176.50] X-Mailer: Zimbra 8.0.7_GA_6021 (ZimbraWebClient - FF35 (Linux)/8.0.7_GA_6021) Thread-Topic: llist: Fix missing lockless_dereference() Thread-Index: oTtwAZYNd4DcnAbXWoF7dIyJmD0oOw== Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3923 Lines: 85 ----- Original Message ----- > From: "Michael Cree" > To: "Mathieu Desnoyers" > Cc: "Greg KH" , linux-alpha@vger.kernel.org, "Richard Henderson" , "Ivan > Kokshaysky" , "Matt Turner" , "Huang Ying" , > linux-kernel@vger.kernel.org, "Paul McKenney" , "David Howells" , > "Pranith Kumar" , stable@vger.kernel.org > Sent: Saturday, February 7, 2015 7:47:29 PM > Subject: Re: [PATCH] llist: Fix missing lockless_dereference() > > On Sat, Feb 07, 2015 at 10:30:44PM +0000, Mathieu Desnoyers wrote: > > > On Fri, Feb 06, 2015 at 09:08:21PM -0500, Mathieu Desnoyers wrote: > > > > A lockless_dereference() appears to be missing in llist_del_first(). > > > > It should only matter for Alpha in practice. > > What could one anticipate to be the symptoms of such a missing > lockless_dereference()? This can trigger corruption of the lockless linked-list, which is used across a few subsystems. AFAIU, the scenario is as follows. Please bear with me, because it's been a while since I've read on the Alpha multi-cache-banks behavior. The list here would be initially non-empty. Initial state of new_last->next is unset (newly allocated); IOW: garbage. CPU A adds a node into the list while CPU B removes a node from the head of the list. CPU A CPU B llist_add_batch() - Stores to new_last->next - implicit full mb before cmpxchg makes the update to CPU A's cache bank containing new_last->next visible to other CPUs before CPU A's cache bank update making head->first visible to other CPUs. - cmpxchg updates head->first = new_first llist_del_first() - entry = load head->first -> here, lack of barrier on Alpha creates a window where CPU B's cache bank can see the updated "head->first", but the cache bank holding the next value did not receive the update yet, since each cache bank have their own channel, which can be independently saturated. - next = load entry->next (dereference entry pointer) - cmpxchg updates head->first = next -> can store unset "next" value into head->first, thus corrupting the linked list. > > The Alpha kernel is behaving pretty well provided one builds a machine > specific kernel and UP. When running an SMP kernel some packages > (most notably the java runtime, but there are a few others) occasionally > lock up in a pthread call --- could be a problem in libc rather then the > kernel. Are those lockups always occasional, or you have ways to reproduce them frequently with stress-tests ? Thanks, Mathieu > > > > Meta-comment, do we really care about Alpha anymore? Is it still > > > consered an "active" arch we support? > > There are a few of us still running recent kernels on Alpha. I am > maintaining the unofficial Debian alpha port at debian-ports, and the > Debian popcon shows about 10 installations of Debian Alpha. > > Cheers > Michael. > -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- 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/