linux-kernel - Re: [PATCH 4/7] ufs: remove the BKL

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Message-Id: <201103021608.31875.arnd@arndb.de>
Date:	Wed, 2 Mar 2011 16:08:31 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Nick Bowler <nbowler@...iptictech.com>
Cc:	linux-kernel@...r.kernel.org,
	Evgeniy Dushistov <dushistov@...l.ru>,
	Nick Piggin <npiggin@...il.com>
Subject: Re: [PATCH 4/7] ufs: remove the BKL

On Wednesday 02 March 2011, Nick Bowler wrote:
> On 2011-03-02 00:13 +0100, Arnd Bergmann wrote:
> > This introduces a new per-superblock mutex in UFS to replace
> > the big kernel lock. I have been careful to avoid nested
> > calls to lock_ufs and to get the lock order right with
> > respect to other mutexes, in particular lock_super.
> > 
> > I did not make any attempt to prove that the big kernel
> > lock is not needed in a particular place in the code,
> > which is very possible.
> > 
> > The code is still only compile-tested,
> 
> This isn't true anymore; I've been running with this patch (well, the
> previous versions thereof) for some time now.  On the other hand, I
> don't use all of this driver's features.

I'll updated the comment. Can I add your Tested-by tag?

> > but it should at least be harmless on non-SMP systems, since the new
> > mutex is not taken on those.
> 
> I think this part of the patch is strange.  It seems like a gratuitous
> difference between SMP/preempt and other systems to #if out the code
> that takes the mutex.  This might make problems with the conversion fly
> under the radar longer because people with older systems won't encounter
> them.

I agree it is strange, but the mutex has some serious performance impact
that I wanted to minimize on the systems where we know it is not needed.
The BKL was only active on those systems, so we know that non-SMP
non-preempt kernels don't need the mutex.

	Arnd
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