Re: [PATCH 6/6] NLM: Add reference counting to lockd

[Christoph Hellwig <hch@infradead.org>]

On Tue, Jan 08, 2008 at 02:33:18PM -0500, Jeff Layton wrote:
> ...and only have lockd exit when the last reference is dropped.
> 
> The problem is this:
> 
> When a lock that a client is blocking on comes free, lockd does this in
> nlmsvc_grant_blocked():
> 
>     nlm_async_call(block->b_call, NLMPROC_GRANTED_MSG, &nlmsvc_grant_ops);
> 
> the callback from this call is nlmsvc_grant_callback(). That function
> does this at the end to wake up lockd:
> 
>     svc_wake_up(block->b_daemon);
> 
> However there is no guarantee that lockd will be up when this happens.
> If someone shuts down or restarts lockd before the async call completes,
> then the b_daemon pointer will point to freed memory and the kernel may
> oops.
> 
> I first noticed this on older kernels and had mistakenly thought that
> newer kernels weren't susceptible, but that's not correct. There's a bit
> of a race to make sure that the nlm_host is bound when the async call is
> done, but I can now reproduce this at will on current kernels.
> 
> This patch is based on Trond's suggestion to add a new reference counter
> to lockd, and only allows lockd to go down when it reaches 0. With this
> change we can't use kthread_stop here. nlmsvc_unlink_block is called by
> lockd and a kthread can't call kthread_stop on itself. So the patch
> changes lockd to check the refcount itself and to return if it goes to
> 0. We do the checking and exit while holding the nlmsvc_mutex to make
> sure that a new lockd is not started until the old one is down.

I don't like this signals/kthread mixture at all.  Why can't we simply
call kthread_stop when the refcount hits zero and keep all the nice
kthread helpers?