Re: [PATCH] sunrpc: remove unnecessary svc_xprt_put

Neil Brown
2010-03-01T04:23:19Z

On Fri, 26 Feb 2010 20:38:25 -0600
Tom Tucker <tom@opengridcomputing.com> wrote:

> Neil Brown wrote:
> > 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?
> >
>
> Here's the commit that I was thinking of...
> 22945e4a1c7454c97f5d8aee1ef526c83fef3223
>
> I think this change adds the bug that you are now fixing. It fixed one
> problem, but added another that you have now resolved.
>
> What do you guys think?

Yes, I see what you are saying.

I agree that commit did fix a problem, but inadvertently introduced a new one.

Thanks,
NeilBrown

>
> Thanks,
> Tom
> > 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
> >
>
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