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bpf: cpumap fix potential lost wake-up problem

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As pointed out by Michael, commit 1c601d829a ("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: 1c601d829a ("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>
This commit is contained in:
Jesper Dangaard Brouer 2017-10-23 19:39:28 +02:00 committed by David S. Miller
parent 7f6b437e9b
commit 31749468c3
1 changed files with 8 additions and 4 deletions
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12
kernel/bpf/cpumap.c
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@ -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();