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libbpf: fix ringbuf synchronization, use EPOLLET #5363

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libbpf: fix ringbuf synchronization, use EPOLLET #5363

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16 changes: 14 additions & 2 deletions tools/lib/bpf/ringbuf.c
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Original file line number Diff line number Diff line change
Expand Up @@ -148,7 +148,7 @@ int ring_buffer__add(struct ring_buffer *rb, int map_fd,
e = &rb->events[rb->ring_cnt];
memset(e, 0, sizeof(*e));

e->events = EPOLLIN;
e->events = EPOLLIN|EPOLLET;
e->data.fd = rb->ring_cnt;
if (epoll_ctl(rb->epoll_fd, EPOLL_CTL_ADD, map_fd, e) < 0) {
err = -errno;
Expand Down Expand Up @@ -260,7 +260,19 @@ static int64_t ringbuf_process_ring(struct ring *r)
cnt++;
}

smp_store_release(r->consumer_pos, cons_pos);
/* This ordering is critical to ensure that an epoll
* notification gets sent in the case where the next
* iteration of this loop discovers that the consumer is
* caught up. If this store were performed using
* RELEASE, it'd be possible for the consumer to fail to
* see an updated producer position and for that
* producer to fail to see this write. By making this
* write SEQ_CST, we know that either the newly produced
* message will be visible to theconsumer, or the
* producer will discover that the consumer is caught
* up, and will ensure a notification is sent.
*/
__atomic_store_n(r->consumer_pos, cons_pos, __ATOMIC_SEQ_CST);
}
} while (got_new_data);
done:
Expand Down