Re: [PATCH 10/10 v7] nfsd: Allows user un-mounting filesystem where nfsd exports base on

[Al Viro <viro@ZenIV.linux.org.uk>]

On Wed, Jul 15, 2015 at 01:49:48PM +1000, NeilBrown wrote:

> > In any case, you have two possibilities - explicit unexport triggering that
> > dput(), etc. and umount(2) triggering the same.  Whoever comes second gets
> > to wait until it's done.  So why not make the point where we commit to
> > unexporting the sucker the place where we do pin_kill()?  And have ->kill()
> > of that thing prevent appearance of new entries, then wait for them to run
> > down.  Which is precisely the same thing you want to happen on umount...
> 
> The "wait for them to run down" part is the sticking point.  We don't
> have any easy way to wait for there to be no more references, so I'd
> really like to use the waiting that pin_kill() already does.

> I want the ->kill function to just unhash the cache entry, and then
> wait for pin_delete() to be called.

pin_remove, presumably?

> The final 'put' on the cache entry calls dput on the dentry and then
> pin_remove().
> 
> The ->kill function can wait for that to happen by calling pin_kill().

> I guess there is no real need for a return value from pin_remove().
> 
> So
>   static void expkey_pin_kill(struct fs_pin *pin)
>   {
>      struct svc_expkey *key = container_of(pin, ....);
> 
>      cache_delete_entry(key->cd, &key->h);


... and another CPU drops the last reference, freeing the sucker.

>      pin_kill(&key->ek_pin); /* recursive call will wait for
>                               * pin_delete() to be called */

... oopsing on the spot.