Re: [PATCH] sunrpc: remove unnecessary svc_xprt_put

Neil Brown
2010-02-27T01:35:46Z

On Fri, 26 Feb 2010 18:40:58 -0600
Tom Tucker <tom@opengridcomputing.com> wrote:

> J. Bruce Fields wrote:
> > On Sat, Feb 27, 2010 at 09:33:40AM +1100, Neil Brown wrote:
> >
> >> [I found this while looking for the current refcount problem
> >> that triggers a warning in svc_recv. This isn't that bug
> >> but is a different refcount bug - NB]
> >>
> >
> >
>
> I seem to recall that we added that reference for a reason. There was
> an issue with unmount while there were deferrals pending. That's why the
> reference was added.
>
> Tom

What reference?
What I (thought I) found was code that was dropping a reference which it
didn't hold. Are you saying that it is supposed to be holding a reference
here, but isn't, or that it really is holding a reference here and I didn't
see it?

And just for completeness, my understanding of the refcounting here is:

A counted references is held on an svc_xprt when:
- a 'struct rqst' refers to it through ->rq_xprt
- a 'cache_deferred_req' refers to it through ->xprt
This only happens while the req is waiting to be
revisited, and is in the hash table and on the lru.
Once the req gets revisited (svc_revisit) ->xprt
is set to NULL and the reference is dropped.
- XPT_DEAD is *not* set. So the refcount is initialised
to '1' to reflect this, and this ref is dropped
when we set XPT_DEAD.
- there are a few transient references in svc_xprt.c
which very clearly have matched 'get' and 'put'.
- svc_find_xprt returns a counted reference. This is
called once in lockd and once in nfsd, and both
calls drop the ref correctly.

Whenever we drop a counted ref that was stored in a pointer, we set that
pointer to NULL.
So if there was a race where two threads both get a reference from a pointer
and then drop that reference, you would expect that slightly different timing
would cause one of those threads to get a NULL from the pointer, dereference
it, and crash. There are no important tests-for-NULL on either of the
pointers in question, so that wouldn't be protecting us from a crash. But
we don't see that crash, so there cannot be a race there.

So: The refcount cannot possibly be zero in svc_recv :-)

I just noticed some slightly odd code later in svc_recv:

if (XPT_LISTENER && XPT_CLOSE) {
...
} else if (XPT_CLOSE) {
...
->xpo_recvfrom()
}
if (XPT_CLOSE) {
...
svc_delete_xprt()
}

So if XPT_CLOSE is set while xpo_recvfrom is being called, which I think
is possible, and if ->xpo_recvfrom returns non-zero, then we end up
processing a request on a dead socket, which doesn't sound like the right
thing to do. I don't think it can cause the present problem, but
it looks wrong. That last 'if' should just be an 'else'.
I guess that would effectively reverse b0401d7253, though - not that
that patch seems entirely right to me - if there is a problem I probably
would have fixed it differently, though I'm not sure how.
So maybe change "if (XPT_CLOSE)" to "if (len <= 0 && XPT_CLOSE)" ???

NeilBrown