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 S1754539AbcCNCjr (ORCPT <rfc822;w@1wt.eu>);
	Sun, 13 Mar 2016 22:39:47 -0400
Received: from mail-pa0-f65.google.com ([209.85.220.65]:33607 "EHLO
	mail-pa0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1754466AbcCNCjd (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Sun, 13 Mar 2016 22:39:33 -0400
Date: Mon, 14 Mar 2016 11:40:55 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Byungchul Park <byungchul.park@lge.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        akpm@linux-foundation.org, mingo@kernel.org,
        linux-kernel@vger.kernel.org, akinobu.mita@gmail.com, jack@suse.cz,
        peter@hurleysoftware.com, tglx@linutronix.de, peterz@infradead.org,
        rostedt@goodmis.org
Subject: Re: [PATCH v6 2/2] printk: Make printing of spin_dump() deferred to
 avoid a deadlock
Message-ID: <20160314024054.GC8074@swordfish>
References: <1457692627-14076-1-git-send-email-byungchul.park@lge.com>
 <1457692627-14076-3-git-send-email-byungchul.park@lge.com>
 <20160314013752.GB8074@swordfish>
 <20160314023030.GE5220@X58A-UD3R>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20160314023030.GE5220@X58A-UD3R>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

Hi,

On (03/14/16 11:30), Byungchul Park wrote:
[..]
> > so can it be
> > 
> > vprintk_emit()
> >  __spin_dump_deferred()
> >   vprintk_deferred()
> >    vprintk_emit()
> >     __spin_dump_deferred()
>       ^^^
>       can be caused by raw_spin_lock(logbug_lock)
> 
> >      vprintk_deferred()
> 
> Yes, it can happen by raw_spin_lock(logbuf_lock) to print warning or error
> message. Are you worrying about an infinite recursion?

yes.

> 1. In the case printing warning, eventually it can fill the buffer without
> actual printing using console.

so the worry is that the CPU that spins on __spin_dump_deferred() has IRQs
disabled and `printk_pending' bit set; but IRQ may never be enabled on this
CPU.

> 2. In the case printing error, the infinite recursion can be prevented by
> debug_locks_off().
> 
> Therefore, no problem.

	-ss