linux-kernel - Re: [GIT, RFC] Killing the Big Kernel Lock

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Message-Id: <201003292318.10212.arnd@arndb.de>
Date:	Mon, 29 Mar 2010 23:18:09 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	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>
Subject: Re: [GIT, RFC] Killing the Big Kernel Lock

On Monday 29 March 2010 19:59:39 Frederic Weisbecker wrote:
> On Mon, Mar 29, 2010 at 12:04:24PM +0100, Arnd Bergmann wrote:
> > On Monday 29 March 2010, Frederic Weisbecker wrote:
> > > On Mon, Mar 29, 2010 at 01:18:48AM +0200, Frederic Weisbecker wrote:
> > > > @@ -1943,7 +1949,7 @@ static ssize_t proc_fdinfo_read(struct file *file, char __user *buf,
> > > >  }
> > > >  
> > > >  static const struct file_operations proc_fdinfo_file_operations = {
> > > > -     .open           = nonseekable_open,
> > > > +     .llseek         = generic_file_llseek,
> > > >       .read           = proc_fdinfo_read,
> > > >  };
> > > >  
> > > > 
> > > > Replacing default_llseek() by generic_file_llseek() as you
> > > > did for most of the other parts is fine.
> > > > 
> > > > But the above changes the semantics as it makes it seekable.
> > > > Why not just keeping it as is? It just ends up in no_llseek().
> > 
> > The default is default_llseek, which uses the BKL and cannot be
> > used if procfs is builtin and the BKL is a module.
> 
> Yeah, but you removed the nonseekable_open and made generic_file_llseek
> in llseek on this one.
> This makes it seekable while it wasn't, changing its ABI.
> It wasn't taking the bkl before that as it was calling
> no_llseek().

Ah, I see what you mean. That change was certainly not intentional
an should be reverted. Thanks for pointing this out.

> > The BKL in procfs is only for proc files that have registered
> > their own .ioctl instead of .unlocked_ioctl method. Converting
> > every file_operations instance to provide an unlocked_ioctl
> > (as one of the other patches does) makes sure that this path
> > is never taken. BTW, there are less than a handful of procfs files
> > that provide an ioctl operation, and those probably should never
> > have been merged.
> 
> 
> There are three of them. I'm going to make them .unlocked_ioctl
> and push the bkl inside, and warn on further uses of .ioctl,
> without applying the bkl there anymore.
> 
> That plus your bkl removal in proc seek, should totally remove the
> bkl from procfs.

Ok

	Arnd
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