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

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Message-ID: <20090417001345.GH26366@ZenIV.linux.org.uk>
Date:	Fri, 17 Apr 2009 01:13:45 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Alessio Igor Bogani <abogani@...ware.it>,
	Alexander Viro <viro@....linux.org.uk>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH -tip] remove the BKL: Replace BKL in mount/umount
	syscalls with a mutex

On Fri, Apr 17, 2009 at 02:01:42AM +0200, Ingo Molnar wrote:
> 
> * Al Viro <viro@...IV.linux.org.uk> wrote:
> 
> > remount is potentially nastier, but then it *is* nasty.  Again, 
> > it's only per-fs stuff, so the obvious first step is taking BKL 
> > down into the instances. It doesn't protect anything in VFS; all 
> > uses are fs internal, so that'll take review of individual 
> > filesystems.
> > 
> > NOTE: do not assume that code in fs/foo/* is correct; "it doesn't 
> > take BKL elsewhere" does _not_ mean that we don't have races.  
> > IOW, the same review ought to look for such beasts and deal with 
> > them.  Mere "oh, no BKL anywhere in that fs" is not enough to 
> > discard the ->remount_fs() instance.
> 
> what kind of races do you mean? Timing sensitive ones that are there 
> just are not easy to trigger with the BKL held?
> 
> Or actual locking interaction between that body of BKL code and all 
> other BKL using code?

Old foo_read_super/foo_write_super/foo_put_super/foo_remount_fs for the
same foo.  IOW, per-driver (and not per-fs - that's taken care of) data
structures.  Arbitrary weird ones.
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