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 S934385AbdHYQVQ (ORCPT <rfc822;w@1wt.eu>);
        Fri, 25 Aug 2017 12:21:16 -0400
Received: from Galois.linutronix.de ([146.0.238.70]:42199 "EHLO
        Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S934151AbdHYQVP (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 25 Aug 2017 12:21:15 -0400
Date: Fri, 25 Aug 2017 18:21:05 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Byungchul Park <max.byungchul.park@gmail.com>
cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Byungchul Park <byungchul.park@lge.com>,
        Peter Zijlstra <peterz@infradead.org>,
        lkml <linux-kernel@vger.kernel.org>, kernel-team@lge.com
Subject: Re: WARNING: possible circular locking dependency detected
In-Reply-To: <CANrsvRNo8KYtLF9Lwj34EAWgC4R5mmTaKb_XuVRAucWTJKau5A@mail.gmail.com>
Message-ID: <alpine.DEB.2.20.1708251813550.1883@nanos>
References: <20170825100304.5cwrlrfwi7f3zcld@pd.tnic> <20170825144755.ms2h2j2xe6gznnqi@linutronix.de> <CANrsvRNo8KYtLF9Lwj34EAWgC4R5mmTaKb_XuVRAucWTJKau5A@mail.gmail.com>
User-Agent: Alpine 2.20 (DEB 67 2015-01-07)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Sat, 26 Aug 2017, Byungchul Park wrote:
> On Fri, Aug 25, 2017 at 11:47 PM, Sebastian Andrzej Siewior
> <bigeasy@linutronix.de> wrote:
> > We hold the sparse_irq_lock lock while waiting for the completion in the
> > CPU-down case and in the CPU-up case we acquire the sparse_irq_lock lock
> > while the other CPU is waiting for the completion.
> > This is not an issue if my interpretation of lockdep here is correct.
> 
> Hello Sebastian,
> 
> I think you parsed the message correctly.
> 
> The message is saying that, for example:
> 
> context A (maybe being up?)
> --
> lock(sparse_irq_lock) // wait for sparse_irq_lock in B to be released
> complete(st->done) // impossible to hit here
> 
> context B (maybe wanting to synchronize with the cpu being up?)
> --
> lock(sparse_irq_lock) // acquired successfully
> wait_for_completion(st->done) // wait for completion of st->done in A
> unlock(sparse_irq_lock) // impossible to hit here
> 
> I cannot check the kernel code at the moment.. I wonder if this scenario is
> impossible. Could you answer it?

Yes, it's impossible because cpu hotplug is globally serialized. So the cpu
down scenario cannot happen in parallel with the cpu up scenario.

Thanks,

	tglx