linux-kernel - Re: Linux 2.6.26-rc4

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Date:	Tue, 10 Jun 2008 12:57:41 +0800
From:	Ian Kent <raven@...maw.net>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Jeff Moyer <jmoyer@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Miklos Szeredi <miklos@...redi.hu>, jesper@...gh.cc,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: Linux 2.6.26-rc4


On Wed, 2008-06-04 at 10:42 +0800, Ian Kent wrote:
> On Wed, 2008-06-04 at 00:00 +0100, Al Viro wrote:
> > On Tue, Jun 03, 2008 at 03:53:36PM -0400, Jeff Moyer wrote:
> > 
> > > autofs4_lookup is called on behalf a process trying to walk into an
> > > automounted directory.  That dentry's d_flags is set to
> > > DCACHE_AUTOFS_PENDING but not hashed.  A waitqueue entry is created,
> > > indexed off of the name of the dentry.  A callout is made to the
> > > automount daemon (via autofs4_wait).
> > > 
> > > The daemon looks up the directory name in its configuration.  If it
> > > finds a valid map entry, it will then create the directory using
> > > sys_mkdir.  The autofs4_lookup call on behalf of the daemon (oz_mode ==
> > > 1) will return NULL, and then the mkdir call will be made.  The
> > > autofs4_mkdir function then instantiates the dentry which, by the way,
> > > is different from the original dentry passed to autofs4_lookup.  (This
> > > dentry also does not get the PENDING flag set, which is a bug addressed
> > > by a patch set that Ian and I have been working on;  specifically, the
> > > idea is to reuse the dentry from the original lookup, but I digress).
> > > 
> > > The daemon then mounts the share on the given directory and issues an
> > > ioctl to wakeup the waiter.  When awakened, the waiter clears the
> > > DCACHE_AUTOFS_PENDING flag, does another lookup of the name in the
> > > dcache and returns that dentry if found.
> > > Later, the dentry gets expired via another ioctl.  That path sets
> > > the AUTOFS_INF_EXPIRING flag in the d_fsdata associated with the dentry.
> > > It then calls out to the daemon to perform the unmount and rmdir.  The
> > > rmdir unhashes the dentry (and places it on the rehash list).
> > > 
> > > The dentry is removed from the rehash list if there was a racing expire
> > > and mount or if the dentry is released.
> > > 
> > > This description is valid for the tree as it stands today.  Ian and I
> > > have been working on fixing some other race conditions which will change
> > > the dentry life cycle (for the better, I hope).
> > 
> > So what happens if new lookup hits between umount and rmdir?
> 
> It will wait for the expire to complete and then wait for a mount
> request to the daemon.

Actually, that explanation is a bit simple minded.

It should wait for the expire in ->revalidate().
Following the expire completion d_invalidate() should return 0, since
the dentry is now unhashed, which causes ->revalidate() to return 0.
do_lookup() should see this and call a ->lookup().

But maybe I've missed something as I'm seeing a problem now.

Ian


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