Date:	Sun, 28 Mar 2010 21:05:50 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
Cc:	Jiri Kosina <jkosina@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Matthew Wilcox <matthew@....cx>,
	Thomas Gleixner <tglx@...utronix.de>, jblunck@...e.de,
	Alan Cox <alan@...ux.intel.com>, Ingo Molnar <mingo@...e.hu>,
	John Kacur <jkacur@...hat.com>
Subject: Re: [GIT, RFC] Killing the Big Kernel Lock

On Sunday 28 March 2010, Stefan Richter wrote:
> Arnd Bergmann wrote:
> > Your patches look good, but it would be helpful to also set .llseek = no_llseek
> > in the file operations, because that is much easier to grep for than
> > only the nonseekable_open. While it's technically a NOP on the presence of
> > nonseekable_open, it will help that I don't accidentally apply my patch on
> > top of yours.
> 
> Sounds like a plan, but (a) if my .llseek = no_llseek and your .llseek =
> default_llseek are not within diff context range, you (or whoever else
> merges mine and yours) only get a compiler warning (Initializer entry
> defined twice) rather than a merge conflict which couldn't be missed,
> (b) there won't be a merge conflict in "BKL removal: mark remaining
> users as 'depends on BKL'".  (c) While I don't mind adding more visual
> clutter to ieee1394/*, I prefer terse coding in firewire/*.
> 
> How about I put my nonseekable_open additions into a release branch and
> send you a pull request after a few days exposure in linux-next?  If you
> do not plan to respin your patch queue soon or at all, I could even let
> you pull a for-arnd branch with a semantically correct merge of yours
> and mine.

I can probably remember this specific one now, but for other people
doing the same on their subsystems, adding no_llseek may help reduce
the need for coordination.

> General thoughts:
> 
> ".llseek = NULL," so far meant "do the Right Thing on lseek() and
> friends, as far as the fs core can tell".  Shouldn't we keep it that
> way?  It's as close to other ".method = NULL," as it can get, which
> either mean "silently skip this method if it doesn't matter" (e.g.
> .flush) or "fail attempts to use this method with a fitting errno" (e.g.
> .write).

My series changes the default from 'default_llseek' to 'generic_file_llseek',
which is almost identical, except for taking the inode mutex instead of the
BKL. Another option that has been discussed before is to make no_llseek
the default, but that might cause more serious problems wiht drivers that
really require seeking.

Since using default_llseek can only ever make a difference if the driver
actually uses the BKL in any other function, I could go through the
patches again and revert those that do no use the BKL anywhere else.

> Of course, as we have already seen with infiniband, firewire, ieee1394,
> .llseek = NULL is ambiguous in practice.  Does the driver really want to
> use default_llseek, or should it rather use no_llseek and/or
> nonseekable_open, or should it even implement a dummy_llseek() { return
> 0; } which avoids the BKL but preserves ABI behaviour?  This needs to be
> resolved for each and every case eventually, regardless of whether or
> when your addition of .llseek = default_llseek enters mainline.

Yes, that also sounds like a good idea. I believe that Jan actually posted
a patch to do that at some point.

	Arnd
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