[riot-notifications] [RIOT-OS/RIOT] drivers/pulse_counter: Use C11 atomics & bugfix (#11432)

Marian Buschsieweke notifications at github.com
Tue Apr 23 14:45:23 CEST 2019

### Contribution description
- Previously the pulse counter used GCC's built-in functions for atomic memory access. This PR changes this to use C11 atomics instead:
    - Use of C11 atomics instead of atomic build-in functions makes the code more portable and readable.
- Previously pulse_counter_reset() did not use an atomic function, so that resets would be racy. (E.g. on the 8-bit AVR platform an 16-bit store is not atomic, unless special care (like using C11 atomics) is taken.)

### Testing procedure
#### Experimental
E.g. run `USEMODULE=pulse_counter make` in `examples/saul` and check if everything is still working.  But it is very plausible that even non-atomic code would work fine for very long time, as the data race here is very unlikely. Therefore, additional testing should be done:

#### Check if atomics are actually used
E.g. compile `examples/saul` for the Arduino Mega2560 (run `USEMODULE=pulse_counter make BOARD=arduino-mega2560` in `examples/saul`). This 8 bit platform is unable to provide lock-free atomic access to 16 bit of memory and, thus, will rely on the `__atomic*()` set of helper functions to perform the atomic memory accesses. Thus

avr-readelf -s bin/arduino-mega2560/saul.a | grep atomic

should yield references to `__atomic_fetch_add_2`, `__atomic_store_2`, `__atomic_exchange_2`, and `__atomic_load_2`.

A correct compilation should contain lines like this

   153: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND __atomic_fetch_add_2
   156: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND __atomic_store_2
   158: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND __atomic_exchange_2
   160: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND __atomic_load_

### Issues/PRs references
You can view, comment on, or merge this pull request online at:


-- Commit Summary --

  * drivers/pulse_counter: Use C11 atomics & bugfix

-- File Changes --

    M drivers/include/pulse_counter.h (3)
    M drivers/pulse_counter/pulse_counter.c (20)

-- Patch Links --


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