Re: [PATCH 3/3] SUNRPC: svc_xprt_enqueue should not refuse to enqueue 'XPT_DEAD' transports

Tom Tucker
2008-12-17T15:35:52Z

Trond Myklebust wrote:
> Aside from being racy (there is nothing preventing someone setting XPT_DEAD
> after the test in svc_xprt_enqueue, and before XPT_BUSY is set), it is
> wrong to assume that transports which have called svc_delete_xprt() might
> not need to be re-enqueued.

This is only true because now you allow transports with XPT_DEAD set to
be enqueued -- yes?

>
> See the list of deferred requests, which is currently never going to
> be cleared if the revisit call happens after svc_delete_xprt(). In this
> case, the deferred request will currently keep a reference to the transport
> forever.
>

I agree this is a possibility and it needs to be fixed. I'm concerned
that the root cause is still there though. I thought the test case was
the client side timing out the connection. Why are there deferred
requests sitting on what is presumably an idle connection?

> The fix should be to allow dead transports to be enqueued in order to clear
> the deferred requests, then change the order of processing in svc_recv() so
> that we pick up deferred requests before we do the XPT_CLOSE processing.
>

Wouldn't it be simpler to clean up any deferred requests in the close
path instead of changing the meaning of XPT_DEAD and dispatching
N-threads to do the same?

> Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
> ---
>
> net/sunrpc/svc_xprt.c | 124 +++++++++++++++++++++++++++----------------------
> 1 files changed, 69 insertions(+), 55 deletions(-)
>
>
> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> index a417064..b54cf84 100644
> --- a/net/sunrpc/svc_xprt.c
> +++ b/net/sunrpc/svc_xprt.c
> @@ -297,10 +297,15 @@ void svc_xprt_enqueue(struct svc_xprt *xprt)
> struct svc_serv *serv = xprt->xpt_server;
> struct svc_pool *pool;
> struct svc_rqst *rqstp;
> + unsigned long flags;
> int cpu;
>
> - if (!(xprt->xpt_flags &
> - ((1<<XPT_CONN)|(1<<XPT_DATA)|(1<<XPT_CLOSE)|(1<<XPT_DEFERRED))))
> + flags = xprt->xpt_flags &
> + (1UL<<XPT_CONN | 1UL<<XPT_DATA | 1UL<<XPT_CLOSE |
> + 1UL<<XPT_DEAD | 1UL<<XPT_DEFERRED);
> + if (flags == 0)
> + return;
> + if ((flags & 1UL<<XPT_DEAD) != 0 && (flags & 1UL<<XPT_DEFERRED) == 0)
> return;
>
> cpu = get_cpu();
> @@ -315,12 +320,6 @@ void svc_xprt_enqueue(struct svc_xprt *xprt)
> "svc_xprt_enqueue: "
> "threads and transports both waiting??\n");
>
> - if (test_bit(XPT_DEAD, &xprt->xpt_flags)) {
> - /* Don't enqueue dead transports */
> - dprintk("svc: transport %p is dead, not enqueued\n", xprt);
> - goto out_unlock;
> - }
> -
> /* Mark transport as busy. It will remain in this state until
> * the provider calls svc_xprt_received. We update XPT_BUSY
> * atomically because it also guards against trying to enqueue
> @@ -566,6 +565,7 @@ static void svc_check_conn_limits(struct svc_serv *serv)
> int svc_recv(struct svc_rqst *rqstp, long timeout)
> {
> struct svc_xprt *xprt = NULL;
> + struct svc_xprt *newxpt;
> struct svc_serv *serv = rqstp->rq_server;
> struct svc_pool *pool = rqstp->rq_pool;
> int len, i;
> @@ -673,62 +673,76 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
> spin_unlock_bh(&pool->sp_lock);
>
> len = 0;
> +
> + /*
> + * Deal with deferred requests first, since they need to be
> + * dequeued and dropped if the transport has been closed.
> + */
> + rqstp->rq_deferred = svc_deferred_dequeue(xprt);
> + if (rqstp->rq_deferred) {
> + svc_xprt_received(xprt);
> + len = svc_deferred_recv(rqstp);
> + }
> +
> if (test_bit(XPT_CLOSE, &xprt->xpt_flags)) {
> dprintk("svc_recv: found XPT_CLOSE\n");
> svc_delete_xprt(xprt);
> - } else if (test_bit(XPT_LISTENER, &xprt->xpt_flags)) {
> - struct svc_xprt *newxpt;
> - newxpt = xprt->xpt_ops->xpo_accept(xprt);
> - if (newxpt) {
> - /*
> - * We know this module_get will succeed because the
> - * listener holds a reference too
> - */
> - __module_get(newxpt->xpt_class->xcl_owner);
> - svc_check_conn_limits(xprt->xpt_server);
> - spin_lock_bh(&serv->sv_lock);
> - set_bit(XPT_TEMP, &newxpt->xpt_flags);
> - list_add(&newxpt->xpt_list, &serv->sv_tempsocks);
> - serv->sv_tmpcnt++;
> - if (serv->sv_temptimer.function == NULL) {
> - /* setup timer to age temp transports */
> - setup_timer(&serv->sv_temptimer,
> - svc_age_temp_xprts,
> - (unsigned long)serv);
> - mod_timer(&serv->sv_temptimer,
> - jiffies + svc_conn_age_period * HZ);
> - }
> - spin_unlock_bh(&serv->sv_lock);
> - svc_xprt_received(newxpt);
> - }
> - svc_xprt_received(xprt);
> - } else {
> - 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
> + goto drop_request;
> + }
> +
> + if (!test_bit(XPT_LISTENER, &xprt->xpt_flags)) {
> + if (len == 0) {
> + dprintk("svc: server %p, pool %u, transport %p, inuse=%d\n",
> + rqstp, pool->sp_id, xprt,
> + atomic_read(&xprt->xpt_ref.refcount));
> len = xprt->xpt_ops->xpo_recvfrom(rqstp);
> +
> + /* No data, incomplete (TCP) read, or accept() */
> + if (len == 0 || len == -EAGAIN)
> + goto drop_request;
> + }
> +
> dprintk("svc: got len=%d\n", len);
> - }
>
> - /* No data, incomplete (TCP) read, or accept() */
> - if (len == 0 || len == -EAGAIN) {
> - rqstp->rq_res.len = 0;
> - svc_xprt_release(rqstp);
> - return -EAGAIN;
> + clear_bit(XPT_OLD, &xprt->xpt_flags);
> +
> + rqstp->rq_secure = svc_port_is_privileged(svc_addr(rqstp));
> + rqstp->rq_chandle.defer = svc_defer;
> +
> + if (serv->sv_stats)
> + serv->sv_stats->netcnt++;
> + return len;
> }
> - clear_bit(XPT_OLD, &xprt->xpt_flags);
>
> - rqstp->rq_secure = svc_port_is_privileged(svc_addr(rqstp));
> - rqstp->rq_chandle.defer = svc_defer;
> + newxpt = xprt->xpt_ops->xpo_accept(xprt);
> + if (newxpt) {
> + /*
> + * We know this module_get will succeed because the
> + * listener holds a reference too
> + */
> + __module_get(newxpt->xpt_class->xcl_owner);
> + svc_check_conn_limits(xprt->xpt_server);
> + spin_lock_bh(&serv->sv_lock);
> + set_bit(XPT_TEMP, &newxpt->xpt_flags);
> + list_add(&newxpt->xpt_list, &serv->sv_tempsocks);
> + serv->sv_tmpcnt++;
> + if (serv->sv_temptimer.function == NULL) {
> + /* setup timer to age temp transports */
> + setup_timer(&serv->sv_temptimer,
> + svc_age_temp_xprts,
> + (unsigned long)serv);
> + mod_timer(&serv->sv_temptimer,
> + jiffies + svc_conn_age_period * HZ);
> + }
> + spin_unlock_bh(&serv->sv_lock);
> + svc_xprt_received(newxpt);
> + }
> + svc_xprt_received(xprt);
>
> - if (serv->sv_stats)
> - serv->sv_stats->netcnt++;
> - return len;
> +drop_request:
> + rqstp->rq_res.len = 0;
> + svc_xprt_release(rqstp);
> + return -EAGAIN;
> }
> EXPORT_SYMBOL(svc_recv);
>
>