The check is unnecessary. In the worst case the semaphore gets unlocked
twice, but it's not an issue because the waiter is only interested in the
first sem_post(), and the semaphore is then not used anymore.
poll() is one of the heavily used methods and thus needs to be optimized
as much as possible.
Test on Pixracer: uorb_tests latency_test
Before:
uORB note: ---------------- LATENCY TEST ------------------
INFO [uorb_tests] mean: 40.4320 us
INFO [uorb_tests] std dev: 1.3466 us
INFO [uorb_tests] min: 39 us
INFO [uorb_tests] max: 57 us
INFO [uorb_tests] missed topic updates: 0
This Patch:
uORB note: ---------------- LATENCY TEST ------------------
INFO [uorb_tests] mean: 31.3480 us
INFO [uorb_tests] std dev: 1.4584 us
INFO [uorb_tests] min: 30 us
INFO [uorb_tests] max: 45 us
INFO [uorb_tests] missed topic updates: 0
- There is an assumption un uORB that PX4_F_RDONLY is not 0.
- But O_RDONLY is 0 on Linux. orb_exists passes 0 to px4_open, which in
uORB::DeviceNode::open leads to an unnecessary allocation of
SubscriberData()
- PX4_F_RDONLY is only used in combination with px4_open, never with open()
However even if it was wrong/unexpected, it did not cause any problems,
just the unnecessary allocation.
Daniel Agar