bpf: cpumap fix potential lost wake-up problem

As pointed out by Michael, commit 1c601d82 ("bpf: cpumap xdp_buff
to skb conversion and allocation") contains a classical example of the
potential lost wake-up problem.

We need to recheck the condition __ptr_ring_empty() after changing
current->state to TASK_INTERRUPTIBLE, this avoids a race between
wake_up_process() and schedule(). After this, a race with
wake_up_process() will simply change the state to TASK_RUNNING, and
the schedule() call not really put us to sleep.

Fixes: 1c601d82 ("bpf: cpumap xdp_buff to skb conversion and allocation")
Reported-by: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/cpumap.c

@@ -288,13 +288,17 @@ static int cpu_map_kthread_run(void *data)
 
 		/* Release CPU reschedule checks */
 		if (__ptr_ring_empty(rcpu->queue)) {
-			__set_current_state(TASK_INTERRUPTIBLE);
-			schedule();
-			sched = 1;
+			set_current_state(TASK_INTERRUPTIBLE);
+			/* Recheck to avoid lost wake-up */
+			if (__ptr_ring_empty(rcpu->queue)) {
+				schedule();
+				sched = 1;
+			} else {
+				__set_current_state(TASK_RUNNING);
+			}
 		} else {
 			sched = cond_resched();
 		}
-		__set_current_state(TASK_RUNNING);
 
 		/* Process packets in rcpu->queue */
 		local_bh_disable();