linux-kernel - Re: Linux 2.6.26-rc4

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Message-Id: <1213088983.3024.49.camel@raven.themaw.net>
Date:	Tue, 10 Jun 2008 17:09:43 +0800
From:	Ian Kent <raven@...maw.net>
To:	Jesper Krogh <jesper@...gh.cc>
Cc:	Al Viro <viro@...IV.linux.org.uk>, Jeff Moyer <jmoyer@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: Linux 2.6.26-rc4


On Tue, 2008-06-10 at 14:40 +0800, Ian Kent wrote:
> On Tue, 2008-06-10 at 08:28 +0200, Jesper Krogh wrote:
> > Ian Kent wrote:
> > > 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.
> > 
> > Ok. Ive been running on the patch for a few days now .. and didn't see
> > any problems. But that being said, I also turned off the --ghost option
> > to autofs so if it actually is the patch or the different codepaths
> > being used, I dont know. Since this is a production system, I'm a bit
> > reluctant to just change a working setup to test it out.
> 
> No need to change anything.

Mmmm .. that comment might not be accurate either.

It's beginning to look like my original approach, a post from back in
Feb 2007, to fix a deadlock bug, wasn't right at all. But we don't
really have time to determine that for sure now as it can take several
days for the bug to trigger.

Ian


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