Re: The recent kref_put warning (was: [PATCH] sunrpc: remove unnecessary svc_xprt_put)

Neil Brown
2010-03-01T05:51:24Z

On Sun, 28 Feb 2010 22:46:47 -0500
"J. Bruce Fields" <bfields@fieldses.org> wrote:

> On Mon, Mar 01, 2010 at 10:57:34AM +1100, Neil Brown wrote:
> > No, you are correct. "return 0" is wrong, it should be "return -EAGAIN",
> > both in the XPT_CLOSE case and the XPT_LISTENER case.
> >
> > I observed that in both those cases, 'len' remained at 0 and we didn't do
> > much else but 'return len', so I optimised.
> > I forgot to factor in:
> >
> > if (len == 0 || len == -EAGAIN) {
> > rqstp->rq_res.len = 0;
> > svc_xprt_release(rqstp);
> > return -EAGAIN;
> > }
> >
> > So the svc_xprt_release needs to be moved in there as well, I'm not sure
> > about the rq_res.len = 0.
> > Maybe that was a bad case of premature-optimisation :-)
> >
> > We should probably leave that last else clause as it is and just have a
> > single return from the function.
>
> OK, so the below is what I'm thinking of sending, after some testing;
> really just a split-up version of your patches (uh, so credits may be
> wrong) with the final cleanup removed:

Credits and code look OK the me, thanks.

>
> 1. remove the extra put from svc_delete_xprt().
> 2,3. Revert 2 problematic patches which caused the oops people
> are seeing.
> 4. Fix the original bug from the rdma series.
>
> And the first 3 will go to stable as well. The 4th might eventually
> too, it just seems less urgent.
>
> I also agree with the cleanup that moves the svc_xprt_received to one
> place, I'm just hoping you won't mind regenerating it against this.

See below.
There is still room to tidy up svc_recv, including getting the xpo_recvfrom
routines to report -EAGAIN when that is what they mean, rather than '0',
but I'm not really happy with what I have so-far so I won't post it yet.

NeilBrown

>From 1e75b9d1232957cd44e0d8ea704c9af431cc85be Mon Sep 17 00:00:00 2001
From: NeilBrown <neilb@suse.de>
Date: Mon, 1 Mar 2010 15:49:11 +1100
Subject: [PATCH] sunrpc: centralise most calls to svc_xprt_received

svc_xprt_received must be called when ->xpo_recvfrom has finished
receiving a message, so that the XPT_BUSY flag will be cleared and
if necessary, requeued for further work.

This call is currently made in each ->xpo_recvfrom function, often
from multiple different point, in each case it is the earliest point
on a particular path where it is known that the protection provided by
XPT_BUSY is no longer needed.

However there are (still) some error paths which do not call
svc_xprt_received, and requiring each ->xpo_recvfrom to make the call
does not encourage robustness.

So: move the svc_xprt_received call to be made just after the
call to ->xpo_recvfrom(), and move it of the various ->xpo_recvfrom
methods.

This means that it may not be called at the earliest possible instant,
but this is unlikely to be a measurable performance issue.

Note that there are still other calls to svc_xprt_received as it is
also needed when an xprt is newly created.

Signed-off-by: NeilBrown <neilb@suse.de>

diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index 6bd41a9..70b74be 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -736,8 +736,10 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
if (rqstp->rq_deferred) {
svc_xprt_received(xprt);
len = svc_deferred_recv(rqstp);
- } else
+ } else {
len = xprt->xpt_ops->xpo_recvfrom(rqstp);
+ svc_xprt_received(xprt);
+ }
dprintk("svc: got len=%d\n", len);
}

diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
index 528efef..7425029 100644
--- a/net/sunrpc/svcsock.c
+++ b/net/sunrpc/svcsock.c
@@ -547,7 +547,6 @@ static int svc_udp_recvfrom(struct svc_rqst *rqstp)
dprintk("svc: recvfrom returned error %d\n", -err);
set_bit(XPT_DATA, &svsk->sk_xprt.xpt_flags);
}
- svc_xprt_received(&svsk->sk_xprt);
return -EAGAIN;
}
len = svc_addr_len(svc_addr(rqstp));
@@ -562,11 +561,6 @@ static int svc_udp_recvfrom(struct svc_rqst *rqstp)
svsk->sk_sk->sk_stamp = skb->tstamp;
set_bit(XPT_DATA, &svsk->sk_xprt.xpt_flags); /* there may be more data... */

- /*
- * Maybe more packets - kick another thread ASAP.
- */
- svc_xprt_received(&svsk->sk_xprt);
-
len = skb->len - sizeof(struct udphdr);
rqstp->rq_arg.len = len;

@@ -917,7 +911,6 @@ static int svc_tcp_recv_record(struct svc_sock *svsk, struct svc_rqst *rqstp)
if (len < want) {
dprintk("svc: short recvfrom while reading record "
"length (%d of %d)\n", len, want);
- svc_xprt_received(&svsk->sk_xprt);
goto err_again; /* record header not complete */
}

@@ -953,7 +946,6 @@ static int svc_tcp_recv_record(struct svc_sock *svsk, struct svc_rqst *rqstp)
if (len < svsk->sk_reclen) {
dprintk("svc: incomplete TCP record (%d of %d)\n",
len, svsk->sk_reclen);
- svc_xprt_received(&svsk->sk_xprt);
goto err_again; /* record not complete */
}
len = svsk->sk_reclen;
@@ -961,14 +953,11 @@ static int svc_tcp_recv_record(struct svc_sock *svsk, struct svc_rqst *rqstp)

return len;
error:
- if (len == -EAGAIN) {
+ if (len == -EAGAIN)
dprintk("RPC: TCP recv_record got EAGAIN\n");
- svc_xprt_received(&svsk->sk_xprt);
- }
return len;
err_delete:
set_bit(XPT_CLOSE, &svsk->sk_xprt.xpt_flags);
- svc_xprt_received(&svsk->sk_xprt);
err_again:
return -EAGAIN;
}
@@ -1110,7 +1099,6 @@ out:
svsk->sk_tcplen = 0;

svc_xprt_copy_addrs(rqstp, &svsk->sk_xprt);
- svc_xprt_received(&svsk->sk_xprt);
if (serv->sv_stats)
serv->sv_stats->nettcpcnt++;

@@ -1119,7 +1107,6 @@ out:
err_again:
if (len == -EAGAIN) {
dprintk("RPC: TCP recvfrom got EAGAIN\n");
- svc_xprt_received(&svsk->sk_xprt);
return len;
}
error:
diff --git a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
index f92e37e..0194de8 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
@@ -566,7 +566,6 @@ static int rdma_read_complete(struct svc_rqst *rqstp,
ret, rqstp->rq_arg.len, rqstp->rq_arg.head[0].iov_base,
rqstp->rq_arg.head[0].iov_len);

- svc_xprt_received(rqstp->rq_xprt);
return ret;
}

@@ -665,7 +664,6 @@ int svc_rdma_recvfrom(struct svc_rqst *rqstp)
rqstp->rq_arg.head[0].iov_len);
rqstp->rq_prot = IPPROTO_MAX;
svc_xprt_copy_addrs(rqstp, xprt);
- svc_xprt_received(xprt);
return ret;

close_out:
@@ -678,6 +676,5 @@ int svc_rdma_recvfrom(struct svc_rqst *rqstp)
*/
set_bit(XPT_CLOSE, &xprt->xpt_flags);
defer:
- svc_xprt_received(xprt);
return 0;
}