Commit 31749468 authored Oct 23, 2017 by Jesper Dangaard Brouer Committed by David S. Miller Oct 24, 2017

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>

parent 7f6b437e

kernel/bpf/cpumap.c

+8 −4

Original line number	Diff line number	Diff line
		@@ -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);
		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();