Date: Thu, 16 Apr 2009 18:49:27 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Christoph Hellwig <hch@infradead.org>,
       Alexander Viro <viro@ftp.linux.org.uk>
Cc: Alessio Igor Bogani <abogani@texware.it>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       LKML <linux-kernel@vger.kernel.org>, Jonathan Corbet <corbet@lwn.net>,
       viro@zeniv.linux.org.uk
Subject: Re: [PATCH -tip] remove the BKL: Replace BKL in mount/umount
	syscalls with a mutex
Message-ID: <20090416164927.GB19281@elte.hu>
References: <1239892078-6039-1-git-send-email-abogani@texware.it> <20090416143626.GA17683@infradead.org>
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* Christoph Hellwig <hch@infradead.org> wrote:

> On Thu, Apr 16, 2009 at 04:27:58PM +0200, Alessio Igor Bogani wrote:
> > Replace ths BKL in sys_mount()/sys_umount() syscalls with a regular mutex.
> 
> Could you try to explain what these actuall try to protect?

They dont really protect anything - the patch is wrong and 
equivalent to a plain removal of the BKL.

The only case we found to ever matter in practice is NFS: it really 
wants to get rid of the BKL in nfsd_get_sb(). So pushing down the 
BKL lock into per filesystems and then removing it from NFS should 
do the trick.

Would be nice to have some tentative Ack (or, a tentative 
non-immediate-NAK) from Al before we go touch a lot of filesystems 
though. Stupid dont-waste-human-effort considerations and stuff.

For us, the much simpler solution would be to drop the BKL in 
nfsd_get_sb() and go on with life without to touch a dozen or so 
filesystems. Alessio, mind trying that too, is it a solution for 
your testcase?

	Ingo
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