linux-kernel - Re: [PATCH] AFS: Implement file locking

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Message-ID: <20070527022502.GB10867@fieldses.org>
Date:	Sat, 26 May 2007 22:25:02 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	David Howells <dhowells@...hat.com>
Cc:	akpm@...l.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] AFS: Implement file locking

On Sun, May 27, 2007 at 12:55:30AM +0100, David Howells wrote:
> J. Bruce Fields <bfields@...ldses.org> wrote:
> 
> > > +	if (!afs_lock_manager) {
> > > +		afs_lock_manager = create_singlethread_workqueue("kafs_lockd");
> > > +		if (!afs_lock_manager)
> > > +			return -ENOMEM;
> > > +	}
> > > +	return 0;
> > 
> > Doesn't this need some locking?
> 
> Oops.  Yes.  It used to be inside the lock_kernel() section, but has since
> escaped.

The BKL wouldn't help, since create_singlethread_workqueue() can sleep.
Or am I missing something?

> > Do you allow upgrades and downgrades?  (Just curious.)
> 
> AFS does not, as far as I know.

So if I request a write lock while holding a read lock, my request will
be denied?

> > > +		/* if we've already got a readlock on the server and no waiting
> > > +		 * writelocks, then we might be able to instantly grant another
> > 
> > Is that comment correct?  (You don't really test for "waiting
> > writelocks", do you?)
> 
> Locally, yes.  'if (list_empty(&vnode->pending_locks))' covers it quite
> handily.

Oops, right, I was overlooking that check.

This is a little strange, though--if there's somebody waiting for a
write lock on an inode (because somebody else already holds a read lock
on it), that shouldn't block requests for read locks.

--b.
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