Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755564Ab3C2NVq (ORCPT ); Fri, 29 Mar 2013 09:21:46 -0400 Received: from mail-ia0-f173.google.com ([209.85.210.173]:59235 "EHLO mail-ia0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754942Ab3C2NVp (ORCPT ); Fri, 29 Mar 2013 09:21:45 -0400 MIME-Version: 1.0 In-Reply-To: <1364551075.5053.82.camel@laptop> References: <1363809337-29718-1-git-send-email-riel@surriel.com> <5150B1C2.8090607@oracle.com> <20130325163844.042a45ba@annuminas.surriel.com> <1364303965.5053.29.camel@laptop> <1364308023.5053.40.camel@laptop> <5151BC78.3030306@surriel.com> <1364373750.5053.54.camel@laptop> <20130328162337.3003ccd4@cuia.bos.redhat.com> <1364551075.5053.82.camel@laptop> Date: Fri, 29 Mar 2013 06:21:44 -0700 Message-ID: Subject: Re: [PATCH v2 -mm -next] ipc,sem: fix lockdep false positive From: Michel Lespinasse To: Peter Zijlstra Cc: Rik van Riel , Sasha Levin , torvalds@linux-foundation.org, davidlohr.bueso@hp.com, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, hhuang@redhat.com, jason.low2@hp.com, lwoodman@redhat.com, chegu_vinod@hp.com, Dave Jones , benisty.e@gmail.com, Ingo Molnar Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1861 Lines: 38 On Fri, Mar 29, 2013 at 2:57 AM, Peter Zijlstra wrote: > On Thu, 2013-03-28 at 19:50 -0700, Michel Lespinasse wrote: >> So, there are a few things I don't like about spin_unlock_wait(): >> >> 1- From a lock ordering point of view, it is strictly equivalent to >> taking the lock and then releasing it - and yet, lockdep won't catch >> any deadlocks that involve spin_unlock_wait. (Not your fault here, >> this should be fixed as a separate change in lockdep. I manually >> looked at the lock ordering here and found it safe). > > Ooh, I never noticed that, but indeed this shouldn't be hard to cure. > >> 2- With the current ticket lock implementation, a stream of lockers >> can starve spin_unlock_wait() forever. Once again, not your fault and >> I suspect this could be fixed - I expect spin_unlock_wait() callers >> actually only want to know that the lock has been passed on, not that >> it actually got to an unlocked state. > > I suppose the question is do we want to fix it or have both semantics > and use lock+unlock where appropriate. We'd have to look at the users to be sure, but I strongly expect they don't need to get in line waiting - it's sufficient to just wait for the head of the queue to move (or for the queue to be empty). There are actually very few users - Just drivers/ata/libata-eh.c for the spin_unlock_wait() function, and a couple more (kernel/task_work.c and kernel/exit.c) for the raw_spin_unlock_wait variant. Guess I'm not the only one to dislike that function :) -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- 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/