Re: [PATCH 1/4] knfsd: Replace lock_kernel with a mutex for nfsd thread startup/shutdown locking.

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Jun 04, 2008 at 05:27:52PM -0400, Jeff Layton wrote:
> On Wed, 4 Jun 2008 17:02:35 -0400
> "J. Bruce Fields" <bfields@fieldses.org> wrote:
> 
> > On Wed, Jun 04, 2008 at 11:03:13AM -0400, Jeff Layton wrote:
> > > diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
> > > index 5ac00c4..d601a77 100644
> > > --- a/fs/nfsd/nfsctl.c
> > > +++ b/fs/nfsd/nfsctl.c
> > ...
> > > @@ -566,14 +574,13 @@ static ssize_t write_versions(struct file *file, char *buf, size_t size)
> > >  	return len;
> > >  }
> > >  
> > > -static ssize_t write_ports(struct file *file, char *buf, size_t size)
> > > +static ssize_t __write_ports(struct file *file, char *buf, size_t size)
> > >  {
> > >  	if (size == 0) {
> > >  		int len = 0;
> > > -		lock_kernel();
> > > +
> > >  		if (nfsd_serv)
> > >  			len = svc_xprt_names(nfsd_serv, buf, 0);
> > > -		unlock_kernel();
> > 
> > svc_xprt_names() has to be prepared to accept NULL as a first parameter
> > (since we've got nothing here any longer to guarantee that nfsd_serv
> > won't change after we've checked it).  And, indeed, it does check for
> > that (with its local copy, which won't change.  So that's OK.  But then
> > could we just ditch this redundant check here?  It's confusing.
> > 
> > Oops, but: what happens if something like this races with svc_destroy,
> > so svc_xprt_names() is passed a pointer to freed memory?
> > 
> 
> We do have a guarantee that nfsd_serv won't change after it's checked
> here. The new nfsd_mutex protects it. write_ports has been renamed to
> __write_ports, and write_ports has been turned into a wrapper that runs
> the entire original function under the nfsd_mutex. We also have nfsd
> hold the nfsd_mutex when svc_exit_thread is called, so svc_destroy
> should also be called while holding it. That should serialize access
> to the nfsd_serv.

Of course, you're right; thanks for setting me straight!

> 
> I think you're correct that we can get rid of the redundant null
> pointer check in __write_ports here though.

OK.--b.