linux-kernel - Re: [PATCH 1/2] BKL: Remove BKL from default

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Message-ID: <20091118175307.GQ21750@bolzano.suse.de>
Date:	Wed, 18 Nov 2009 18:53:07 +0100
From:	Jan Blunck <jblunck@...e.de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	linux-fsdevel@...r.kernel.org,
	Linux-Kernel Mailinglist <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, jkacur@...hat.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnd Bergmann <arnd@...db.de>,
	Christoph Hellwig <hch@...radead.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 1/2] BKL: Remove BKL from default_llseek()

On Wed, Nov 18, Alan Cox wrote:

> > Using the BKL in llseek() does not protect the inode's i_size from
> > modification since the i_size is protected by a seqlock nowadays. Since
> > default_llseek() is already using the i_size_read() wrapper it is not the
> > BKL which is serializing the access here.
> > The access to file->f_pos is not protected by the BKL either since its
> > access in vfs_write()/vfs_read() is not protected by any lock. If the BKL
> > is not protecting anything here it can clearly get removed.
> 
> No. Your logic is flawed
> 
> The BKL is protected something here - it protects the change of offset
> with respect to other BKL users within drivers. The question is what if
> anything in any other driver code depends upon the BKL and uses it to
> protect f_pos. Probably very little if anything but a grep for f_pos
> through the drivers might not be a bad idea before assuming this. Very
> few touch f_pos except in their own llseek method.

As I said, f_pos is changed without holding BKL in the VFS already. Therefore
even if the driver tries to protect f_pos by holding the BKL it is racing
against concurrent read()/write() anyway on f_pos.

Regards,
	Jan

-- 
Jan Blunck <jblunck@...e.de>
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