[NET]: Fix race between poll_napi() and net_rx_action()

netpoll_poll_lock() synchronizes the ->poll() invocation
code paths, but once we have the lock we have to make
sure that NAPI_STATE_SCHED is still set.  Otherwise we
get:

	cpu 0			cpu 1

	net_rx_action()		poll_napi()
	netpoll_poll_lock()	... spin on ->poll_lock
	->poll()
	  netif_rx_complete
	netpoll_poll_unlock()	acquire ->poll_lock()
				->poll()
				 netif_rx_complete()
				 CRASH

Based upon a bug report from Tina Yang.

Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 37 insertions, 10 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 853c8b575f1d..02e7d8377c4a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2172,7 +2172,15 @@ static void net_rx_action(struct softirq_action *h)
 
                 weight = n->weight;
 
-                work = n->poll(n, weight);
+                /* This NAPI_STATE_SCHED test is for avoiding a race
+                 * with netpoll's poll_napi().  Only the entity which
+                 * obtains the lock and sees NAPI_STATE_SCHED set will
+                 * actually make the ->poll() call.  Therefore we avoid
+                 * accidently calling ->poll() when NAPI is not scheduled.
+                 */
+                work = 0;
+                if (test_bit(NAPI_STATE_SCHED, &n->state))
+                        work = n->poll(n, weight);
 
                 WARN_ON_ONCE(work > weight);
 
diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index bf8d18f1b013..c499b5c69bed 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -116,6 +116,29 @@ static __sum16 checksum_udp(struct sk_buff *skb, struct udphdr *uh,
  * network adapter, forcing superfluous retries and possibly timeouts.
  * Thus, we set our budget to greater than 1.
  */
+static int poll_one_napi(struct netpoll_info *npinfo,
+                         struct napi_struct *napi, int budget)
+{
+        int work;
+
+        /* net_rx_action's ->poll() invocations and our's are
+         * synchronized by this test which is only made while
+         * holding the napi->poll_lock.
+         */
+        if (!test_bit(NAPI_STATE_SCHED, &napi->state))
+                return budget;
+
+        npinfo->rx_flags |= NETPOLL_RX_DROP;
+        atomic_inc(&trapped);
+
+        work = napi->poll(napi, budget);
+
+        atomic_dec(&trapped);
+        npinfo->rx_flags &= ~NETPOLL_RX_DROP;
+
+        return budget - work;
+}
+
 static void poll_napi(struct netpoll *np)
 {
         struct netpoll_info *npinfo = np->dev->npinfo;
@@ -123,17 +146,13 @@ static void poll_napi(struct netpoll *np)
         int budget = 16;
 
         list_for_each_entry(napi, &np->dev->napi_list, dev_list) {
-                if (test_bit(NAPI_STATE_SCHED, &napi->state) &&
-                    napi->poll_owner != smp_processor_id() &&
+                if (napi->poll_owner != smp_processor_id() &&
                     spin_trylock(&napi->poll_lock)) {
-                        npinfo->rx_flags |= NETPOLL_RX_DROP;
-                        atomic_inc(&trapped);
-
-                        napi->poll(napi, budget);
-
-                        atomic_dec(&trapped);
-                        npinfo->rx_flags &= ~NETPOLL_RX_DROP;
+                        budget = poll_one_napi(npinfo, napi, budget);
                         spin_unlock(&napi->poll_lock);
+
+                        if (!budget)
+                                break;
                 }
         }
 }