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

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Message-ID: <20090417173422.GJ26366@ZenIV.linux.org.uk>
Date:	Fri, 17 Apr 2009 18:34:22 +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>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH -tip] remove the BKL: Replace BKL in mount/umount
	syscalls with a mutex

On Fri, Apr 17, 2009 at 06:56:43PM +0200, Ingo Molnar wrote:

> Stupid question regarding c): wouldnt such data structures go via 
> the VFS - which you said was free of BKL constraints? Or are there 
> interconnected private data structures between certain types of 
> closely related filesystems that the VFS does not know about? (and 
> hence might have BKL assumptions)

Shared between different instances of this fs type.  E.g. if FAT had
its hashtable not per-superblock (as it is now) but system-wide (as it
used to be prior to [PATCH] FAT: the inode hash from per module to per sb),
it would be suspicious.  In this particular case, it was OK from the
very beginning (I'd used a system-wide spinlock to protect it when it
had been introduced and per-superblock splitup made that spinlock per-sb
along with the hash table).

IOW, I wouldn't particulary worry about interactions between different
fs _types_ - it's interactions between the different fs _instances_
within the same driver that are likely sources of PITA.

ObOtherQuestion: no, I don't think that bdev open and bdev ioctls situation
is related to this group of BKL users.  sb_set_blocksize() might, in principle,
want a memory barrier of some kind, but that's it.
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