linux-kernel - Re: [PATCH RFC] [PATCH] drivers/scsi/ch.c: Remove BKL in ch

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Message-ID: <20091011082917.6ebbd810@infradead.org>
Date:	Sun, 11 Oct 2009 08:29:17 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	John Kacur <jkacur@...hat.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Christoph Hellwig <hch@...radead.org>,
	Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vincent Sanders <vince@...tec.co.uk>,
	linux-scsi@...r.kernel.org
Subject: Re: [PATCH RFC] [PATCH] drivers/scsi/ch.c: Remove BKL in ch_open

On Sun, 11 Oct 2009 15:37:09 +0100
Alan Cox <alan@...rguk.ukuu.org.uk> wrote:

> On Sun, 11 Oct 2009 15:45:16 +0200 (CEST)
> John Kacur <jkacur@...hat.com> wrote:
> 
> [linux-scsi cc'd]
> 
> > Locking in ch_open is covered by the spin_lock, it serializes the
> > calls to idr_find and scsi_device_get. The BKL appears redundant to
> > me here.
> 
> I'm not so sure. In fact there are some quite umm interesting
> questions about this code, and some of them are shared with other
> modules too.
> 
> Consider the following sequence
> 
> 		CPU1				CPU2
> 	register_chrdev
> 		ok
> 						open device
> 						takes lock


but open does not take the BKL, so the BKL is not protecting you at
all against this..



-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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