From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751238AbdHXFGo (ORCPT <rfc822;w@1wt.eu>);
        Thu, 24 Aug 2017 01:06:44 -0400
Received: from LGEAMRELO13.lge.com ([156.147.23.53]:45416 "EHLO
        lgeamrelo13.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751140AbdHXFGm (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 24 Aug 2017 01:06:42 -0400
X-Original-SENDERIP: 156.147.1.125
X-Original-MAILFROM: byungchul.park@lge.com
X-Original-SENDERIP: 10.177.222.33
X-Original-MAILFROM: byungchul.park@lge.com
Date: Thu, 24 Aug 2017 14:06:39 +0900
From: Byungchul Park <byungchul.park@lge.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>, mingo@kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@lge.com,
        Arnaldo Carvalho de Melo <acme@kernel.org>, Tejun Heo <tj@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH v3 1/3] lockdep: Make LOCKDEP_CROSSRELEASE configs all
 part of PROVE_LOCKING
Message-ID: <20170824050639.GD6772@X58A-UD3R>
References: <1502960261-16206-1-git-send-email-byungchul.park@lge.com>
 <20170821154600.asyzqs2zg6w6o4pg@hirez.programming.kicks-ass.net>
 <20170822054603.GT10621@dastard>
 <20170822090603.vufl5ncoequ5ag6x@hirez.programming.kicks-ass.net>
 <20170823023118.GC3108@X58A-UD3R>
 <20170823104648.ltpwbiprzconz7ef@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170823104648.ltpwbiprzconz7ef@hirez.programming.kicks-ass.net>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Aug 23, 2017 at 12:46:48PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 23, 2017 at 11:31:18AM +0900, Byungchul Park wrote:
> > On Tue, Aug 22, 2017 at 11:06:03AM +0200, Peter Zijlstra wrote:
> 
> > Currently, we do the following in process_one_work(),
> > 
> > lockdep_map_acquire for a workqueue
> > lockdep_map_acquire for a work
> > 
> > But IMHO it should be,
> > 
> > lockdep_map_acquire for a pair of workqueue and work.
> > 
> > Right?
> 
> No, it is right. We need the two 'locks'.
> 
> The work one is for flush_work(), the workqueue one is for
> flush_workqueue().
> 
> Just like how flush_work() must not depend on any lock taken inside the
> work, flush_workqueue() callers must not hold any lock acquired inside
> any work ran inside the workqueue. This cannot be done with a single
> 'lock'.

Thank you for explanation.

> The reason flush_work() also depends on the wq 'lock' is because doing
> flush_work() from inside work is a possible problem for single threaded
> workqueues and workqueues with a rescuer.
>  
> > > Agreed. The interaction with workqueues is buggered.
> > 
> > I think original uses of lockdep_map were already wrong. I mean it's
> > not a problem of newly introduced code.
> 
> Not wrong per-se, the new code certainly places more constraints on it.

"the new code places more constraints on it" is just the right expression.