Re: nfs: infinite loop in fcntl(F_SETLKW)

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

On Sun, Apr 13, 2008 at 10:28:08AM +0200, Miklos Szeredi wrote:
> > Apologies, that was indeed a behavioral change introduced in a commit
> > that claimed just to be shuffling code around.
> 
> Another complaint about this series: using EINPROGRESS to signal
> asynchronous locking looks really fishy.  How does the filesystem
> know, that the caller wants to do async locking?

The caller sets a fl_grant callback.  But I guess the interesting
question is how the caller knows that the filesystem is really going to
return the results asynchronously:

> How do we make sure,
> that the filesystem (like fuse or 9p, which "blindly" return the error
> from the server) doesn't return EINPROGRESS even when it's _not_ doing
> an asynchronous lock?

Right, there's no safeguard there--if fuse returns EINPROGRESS, then
we'll wait for a grant callback that's not going to come.  It should
time out, so that's not a total disaster, but still.

Anyway, I'm not sure what to do about that.

> 
> I think it would have been much cleaner to have a completely separate
> interface for async locking, instead of trying to cram that into
> f_op->lock().

Maybe so.  ->lock() had quite a bit crammed into it even before this.

> Would that be possible to fix now?  Or at least make EINPROGRESS a
> kernel-internal error value (>512), to make it that it has a special
> meaning for the _kernel only_?

Perhaps so.

The behavior of lockd will still depend to some degree on the exact
error returned from the filesystem--e.g. if you return -EAGAIN from
->lock() without later calling ->fl_grant() it will cause some
unexpected delays.  (Though again clients will eventually give up and
poll for the lock.)

--b.