Re: Linux 2.6.26-rc4

[Jesper Krogh <jesper@krogh.cc>]

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.

Jesper
-- 
Jesper