linux-kernel - Re: [PATCH -tip] remove the BKL: Replace BKL in mount/umount syscalls with a mutex

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Date:	Thu, 16 Apr 2009 18:49:27 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Hellwig <hch@...radead.org>,
	Alexander Viro <viro@....linux.org.uk>
Cc:	Alessio Igor Bogani <abogani@...ware.it>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	Jonathan Corbet <corbet@....net>, viro@...iv.linux.org.uk
Subject: Re: [PATCH -tip] remove the BKL: Replace BKL in mount/umount
	syscalls with a mutex

* Christoph Hellwig <hch@...radead.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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