linux-kernel - Re: [GIT PATCH] TTY patches for 2.6.33-git

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Date:	Sun, 13 Dec 2009 20:04:13 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: [GIT PATCH] TTY patches for 2.6.33-git

On Sun, Dec 13, 2009 at 07:17:26PM +0100, Ingo Molnar wrote:
> 
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > 
> > 
> > On Sun, 13 Dec 2009, Ingo Molnar wrote:
> > > 
> > > In the last ~4.5 years:
> > > 
> > >  earth4:~/tip> git checkout v2.6.12
> > >  Date:   Fri Jun 17 12:48:29 2005 -0700
> > >  earth4:~/tip> git grep -w unlock_kernel | wc -l
> > >  713
> > > 
> > >  earth4:~/tip> git checkout linus
> > >  Date:   Fri Dec 11 20:58:20 2009 -0800
> > >  earth4:~/tip> git grep -w unlock_kernel | wc -l
> > >  841
> > 
> > Git hint of the day: you don't need to check out a kernel to do "git 
> > grep".
> > 
> > Do this:
> > 
> > 	git grep -w unlock_kernel v2.6.12 | wc
> > 
> > and it will JustWork(tm).
> 
> /me adds it to the metal toolbox
> 
> > > Also, a lot of BKL use was hidden before, and due to the BKL removal 
> > > activities (by Thomas, Frederic, Jon, Alan and others) the remaining BKL using 
> > > sites are a lot more well defined, a lot more isolated and thus a lot more 
> > > removable than ever before.
> > 
> > That's the main thing. We've been pushing them down a lot.
> > 
> > We still have a few annoying core ones, though. I hate the execve() and file 
> > locking ones.
> 
> When we did the BKL-as-a-mutex trick and let lockdep loose on it, three areas 
> were particularly tricky: tty, reiser3 and NFS. tty and reiserfs should be ok 
> now, but i havent seen much activity on the NFS front.
> 
> 	Ingo


Nfs was a problem in the BKL-as-a-mutex tree because it is acquired
recursively and then locked up. And I suspect this recursion happens
only on mount time because vfs acquires it too there.

But looking at it more closely, it doesn't look that dramatic at
a first glance.

git-grep unlock_kernel fs/nfs

fs/nfs/callback.c:      unlock_kernel();
fs/nfs/callback.c:      unlock_kernel();

In nfs4_callback_svc() it embraces the socket
listening/processing callback thread.

svc_recv() might sleep so the bkl can be
lost in the middle.

And then svc_process().

This doesn't seem to protect anything there.

In nfs4_callback_svc() it's about the same thing, plus
a list manipulation but protected by a spinlock.


fs/nfs/delegation.c:            unlock_kernel();
fs/nfs/delegation.c:    unlock_kernel();
fs/nfs/nfs4state.c:             unlock_kernel();
fs/nfs/nfs4state.c:     unlock_kernel();



In the above cases we have the following comment:

/* Protect inode->i_flock using the BKL */

And really it doesn't seem to protect anything else,
fortunately it is acquired in a short path.


fs/nfs/super.c: unlock_kernel();
fs/nfs/super.c: unlock_kernel();

Protect the mount/unmount path, a bit trickier there.

But really that looks way much easier to fix than it was
with reiserfs.

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