[net-next PATCH] bpf: cpumap fix potential lost wake-up problem

[Jesper Dangaard Brouer]

As pointed out by Michael, commit 1c601d829ab0 ("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: 1c601d829ab0 ("bpf: cpumap xdp_buff to skb conversion and allocation")
Reported-by: "Michael S. Tsirkin" <m...@redhat.com>
---
 kernel/bpf/cpumap.c |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
index b4358d84ddf1..86e29cbf7827 100644
--- a/kernel/bpf/cpumap.c
+++ b/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();