Re: The recent kref_put warning

Tom Tucker
2010-02-28T00:47:04Z

Makes sense to me.

Neil Brown wrote:
> On Sat, 27 Feb 2010 12:35:37 +1100
> Neil Brown <neilb@suse.de> wrote:
>
>
>> 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)" ???
>>
>
> OK, I think I've nailed it. I think b0401d7253 is the culprit.
> Now let me see if I can convince you (and me).
>
> Firstly, why is this patch wrong.
> It claims:
>
> sunrpc: "Move close processing to a single place"
> (d7979ae4a050a45b78af51832475001b68263d2a) moved the
> close processing before the recvfrom method. This may
> cause the close processing never to execute. So this
> patch moves it to the right place.
>
> The referenced commit did *not* move the close processing before the
> recvfrom method - it was already there. The close processing was previously
> at the top of the individual recvfrom methods. It was split out and common
> code with placed before the now-slightly-reduced recvfrom methods.
> This is functionally a null change.
>
> However that doesn't explain why sometimes "the close processing [would]]
> never .. execute".
> The reason for this is subtle. One the changes in commit d7979ae4a is
>
> err_delete:
> - svc_delete_socket(svsk);
> + set_bit(SK_CLOSE, &svsk->sk_flags);
> return -EAGAIN;
>
> This is insufficient. The recvfrom methods must always call svc_xprt_received
> on completion so that the socket gets re-queued if there is any more work to
> do. This particular path did not make that call because it actually
> destroyed the svsk, making requeue pointless. When the svc_delete_socket was
> change to just set a bit, we should have added a call to svc_xprt_received,
> but we didn't. Sorry. As I said, it was subtle.
>
> So how is the b0401d7253 patch causing a problem?
>
> svc_tcp_state_change - which can be called at any time - sets XPT_CLOSE.
> If this happens while svc_tcp_recvfrom is running and before one of the
> calls to svc_xprt_received, then svc_xprt_received will requeue the svsk for
> further processing (to handle the close).
> As soon a svc_tcp_recvfrom completes, svc_recv will notice that XPT_CLOSE is
> set and will close the socket, dropping the last refcount. Subsequently the
> thread which the socket was queued to wakes up, calls svc_recv, and triggers
> the warning.
>
> So the fix I propose is:
> - make the XPT_CLOSE case in svc_recv once more exclusive with the
> ->recvfrom case
> - make sure all paths out of all recvfrom methods call svc_xprt_received.
> Maybe it should be called after the ->xpo_recvfrom call instead.
>
> So something like this? I've made quite a few changes here - it might be
> worth splitting them up. One worth noting is that we now don't re-queue a udp
> socket at the earliest opportunity, but possibly do a
> csum_partial_copy_to_xdr before the requeue which could reduce performance
> slightly with udp on a multiprocessor. I have no idea what the actual
> performance effect would be, but I think it makes the code a lot more robust
> (move the svc_xprt_received to just one place).
>
> NeilBrown
>
> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> index 4f30336..2d99fb8 100644
> --- a/net/sunrpc/svc_xprt.c
> +++ b/net/sunrpc/svc_xprt.c
> @@ -699,8 +699,12 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
> spin_unlock_bh(&pool->sp_lock);
>
> len = 0;
> - if (test_bit(XPT_LISTENER, &xprt->xpt_flags) &&
> - !test_bit(XPT_CLOSE, &xprt->xpt_flags)) {
> +
> + if (test_bit(XPT_CLOSE, &xprt->xpt_flags)) {
> + dprintk("svc_recv: found XPT_CLOSE\n");
> + svc_delete_xprt(xprt);
> + return 0;
> + } else if (test_bit(XPT_LISTENER, &xprt->xpt_flags)) {
> struct svc_xprt *newxpt;
> newxpt = xprt->xpt_ops->xpo_accept(xprt);
> if (newxpt) {
> @@ -726,23 +730,19 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
> svc_xprt_received(newxpt);
> }
> svc_xprt_received(xprt);
> - } else if (!test_bit(XPT_CLOSE, &xprt->xpt_flags)) {
> - dprintk("svc: server %p, pool %u, transport %p, inuse=%d\n",
> - rqstp, pool->sp_id, xprt,
> - atomic_read(&xprt->xpt_ref.refcount));
> - rqstp->rq_deferred = svc_deferred_dequeue(xprt);
> - if (rqstp->rq_deferred) {
> - svc_xprt_received(xprt);
> - len = svc_deferred_recv(rqstp);
> - } else
> - len = xprt->xpt_ops->xpo_recvfrom(rqstp);
> - dprintk("svc: got len=%d\n", len);
> + return 0;
> }
>
> - if (test_bit(XPT_CLOSE, &xprt->xpt_flags)) {
> - dprintk("svc_recv: found XPT_CLOSE\n");
> - svc_delete_xprt(xprt);
> - }
> + dprintk("svc: server %p, pool %u, transport %p, inuse=%d\n",
> + rqstp, pool->sp_id, xprt,
> + atomic_read(&xprt->xpt_ref.refcount));
> + rqstp->rq_deferred = svc_deferred_dequeue(xprt);
> + if (rqstp->rq_deferred)
> + len = svc_deferred_recv(rqstp);
> + else
> + len = xprt->xpt_ops->xpo_recvfrom(rqstp);
> + dprintk("svc: got len=%d\n", len);
> + svc_xprt_received(xprt);
>
> /* No data, incomplete (TCP) read, or accept() */
> if (len == 0 || len == -EAGAIN) {
> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
> index 870929e..22d9904 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;
> @@ -961,10 +953,9 @@ 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);
> @@ -1109,18 +1100,14 @@ 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++;
>
> return len;
>
> err_again:
> - if (len == -EAGAIN) {
> + if (len == -EAGAIN)
> dprintk("RPC: TCP recvfrom got EAGAIN\n");
> - svc_xprt_received(&svsk->sk_xprt);
> - return len;
> - }
> error:
> if (len != -EAGAIN) {
> printk(KERN_NOTICE "%s: recvfrom returned errno %d\n",
> 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;
> }
> --
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