linux-kernel - Re: [PATCH] Remove BKL from fs/locks.c

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Message-Id: <201009142239.02904.arnd@arndb.de>
Date:	Tue, 14 Sep 2010 22:39:02 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Matthew Wilcox <willy@...ux.intel.com>,
	Christoph Hellwig <hch@....de>,
	Trond Myklebust <trond.myklebust@....uio.no>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, John Kacur <jkacur@...hat.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] Remove BKL from fs/locks.c

On Tuesday 14 September 2010 22:20:22 Linus Torvalds wrote:
> End result: anybody who uses the old exports will just not link at
> all, and all in-kernel users (ie lockd etc) will now very explicitly
> have that xyz_bkl() use.
> 
> After that one release, we can re-introduce the old names without the
> bkl, and let filesystems and other subsystems just decide that they
> don't want the bkl version one by one. So that first phase would be
> basically a no-op with an interface rename just so that the rest could
> then be done piece-meal.

Hmm, maybe I'm misunderstanding part of that plan, but I think it
won't work because the BKL in there seems to protect all struct file_lock
instances and they are passed around to other code, most importantly
lockd. The file_locks themselves can be accessed both using sys_flock()
etc locally and using afs/nfs/coda/... exported file systems from other
systems.

AFAICT we could end up with lockd changing data under BKL while a sys_flock()
accesses the same data under file_lock_lock, with no mutual exclusion
between the two.

	Arnd
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