linux-kernel - Re: [PATCH 1/5] VFS: Do (nearly) lockless generic_file

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Date:	Fri, 17 Jun 2011 23:18:38 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	viro@...iv.linux.org.uk, hch@...radead.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/5] VFS: Do (nearly) lockless generic_file_llseek

On Friday 17 June 2011 19:42:39 Andi Kleen wrote:
> On Fri, Jun 17, 2011 at 02:40:14PM +0200, Arnd Bergmann wrote:
> > On Thursday 16 June 2011 23:07:18 Andi Kleen wrote:
> > > So still need a lock, but can use a cheap local one.
> > > 
> > > This patch implements this new scheme in generic_file_llseek.
> > > I dropped generic_file_llseek_unlocked and changed all callers.
> > 
> > What about default_llseek? If you change generic_file_llseek, you should
> > probably change that, too.
> 
> That's used by a lot of drivers, and since I cannot audit them
> all I chose to be conservative. not sure it's ever performance
> critical anyways?

Probably not. Note that all uses of default_llseek were introduced
in the conversion from an implicit default_llseek when they did not
set the operation before the BKL conversion. There were also patches
floating around to merge generic_file_llseek and default_llseek.

There was not a single case of a driver that relied on the BKL to
lock against default_llseek before I changed it to i_mutex.

	Arnd
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