Re: [PATCH] sunrpc: remove unnecessary svc_xprt_put

Tom Tucker
2010-02-27T00:40:59Z

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

> And thanks very much for looking into that, I'm worried.... Seems to
> have appeared some time between v2.6.31 and v2.6.32.2. On a quick skim
> commits in that range that struck me as worth a second look included
> 8f55f3c0a013, b0401d725334, and the 4.1 backchannel patches (3ddc8bf5f3
> and preceding).
>
> Oh, and I also have some very rough notes from when I looked at this
> before, in case there's anything useful.
>
> --b.
>
> Re: 2.6.32.2 - WARNING: at lib/kref.c:43 kref_get+0x,23/0x2b()
>
> Seen on: 2.6.32.2, 2.6.32.6, 2.6.32.8; probably was OK on 2.6.29.6 and
> 2.6.31.
>
> Is the warning actually warning about anything that's a problem, or can
> that counter by zero by design? Yes, it's actually a problem.
>
> Is probably svc_xprt_get(xprt) in svc_recv() (only obvious kref_get I
> found on a quick glance through svc_recv).
> Double-check:
> svc_recv+0x305/0x7e6
> Note next bug is on putting a socket (that we probably
> shouldn't have!?):
> - BUG_ON(inode->i_state == I_CLEAR).
> - Implies clear_inode() was previously called on
> it.
> - stack includes kref_put() call in
> svc_xprt_release, which is indeed put of same
> xpt_ref field that svc_xprt_get() gets.
>
> So, most probably explanation:
> - We still had a dangling reference to an xprt after putting
> one. So we ended up doing another get/put pair on it later
> and trying to free the same socket twice.
>
> So, plan: look for svc_xprt_puts (after checking for other stray uses of
> xpt_ref) and verify that they're all legit. And gets while we're at it:
>
> Ignore svc_rdma for now. Those reporters that answered weren't
> using rdma.
>
> Most puts outside of rdma are in svc_xprt.c:
>
> - svc_xprt_release (unconditional): 0 to caller (put matched
> with removal from rq_xprt)
> - svc_check_conn_limits(): 0 to caller
> - takes an xprt off a sv_tempsocks list, gets it (and
> sets XPT_CLOSE) before dropping sv_lock, then enqueues
> and puts. (Note: enqueue will get, and assign to
> rq_xprt, if thread found.)
> - svc_age_temp_xprts: 0 to caller
> - same pattern as svc_check_conn_limits().
> - svc_delete_xprt: 0 to caller (put matched with removal from
> xpt_list)
> - if test_and_set_bit of XPT_DEAD succeeds, will
> svc_xprt_put(), after calling xpo_detach, then (under
> sv_lock) removing xpt_list.
> - ALSO unconditionally puts once for each deferred
> request it finds associated with this request. Is
> that right? Yup: svc_defer() gets on success, when
> assigning dr->xprt.
> - svc_close_xprt:
> - sets XPT_CLOSE, then if test_and_set_bit of XPT_BUSY
> succeeds, gets xprt, deletes, clears BUSY, puts.
> - revisit:
> - puts associated xprt unconditionally.
>
> Also some puts are in fs/nfsd/nfsctl.c, fs/nfsd/nfs4state.c,
> fs/lockd/svc.c:
>
> nfsctl.c:
> ifs/nfsd/nfsctl.c:__write_ports_delxprt():
> - svc_find_xprt() gets a reference; if found:
> svc_close_xprt, svc_xprt_put. OK.
> nfs4state.c:
> free_client: svc_xprt_put(clp->cl_cb_xprt);
> Looks basically correct: we take reference when we
> assign that in nfsd4_create_session.
>
> Hm. Note we copy pointer to clp->cl_cb_xprt without
> taking reference? The client holds a reference,
> though. Looking at cb_xprt use in client xprt code, I
> can't see any references taken or dropped. This all
> looks fine.
>
> lockd/svc.c: create_lock_listener() looks innocuous.
>