sys/{tsrb, oorb}: Thread-safe ringbuffers

[maribu]

Contribution description

• Rename the current tsrb implementation to oorb
  • This highlights that this is a one reader, one writer thread-safe ringbuffer
• Re-add the original tsrb code, but insert irq_disable() ... irq_restore() where needed to provide thready-safety for multiple readers and writers
  • Existing users will just use this instead, as the API is untouched
• Add a note of warning to oorb
  • This issues this warns about are hopefully addressed soonish

Testing procedure

E.g. stdio over UART on STM32 should still work. And now concurrent stdio should not result in corrupting the output (only in mixing it).

Issues/PRs references

Addresses #9882. (But the same issues still apply to the new oorb. However, with the documentation pointing those issues out, I think we can consider them as features :-D. And those "features" should be addressed in a follow up soonish.)

[benpicco]

I wonder if the TSRB could just do e.g.

static inline int tsrb_empty(const oorb_t *rb)
{
    int retval;
    unsigned status = irq_disable();
    retval = oorb_empty(rb);
    irq_restore(status);
    return retval;
}

to avoid code duplication

[benpicco]

Do those need to be volatie?`

[maribu]

No, those should be atomic. Or more precisely, semi-atomic :-D

(As this is the two-threads-one-buffer mode, only the reader will write to reads and only the writer will write to writes. So as long as the read and the write access to them is atomically, everything is fine. This means explicitly that a read-modify-write operation could consist of three operations with only the read and write being atomic. Real atomic accesses would make the whole read-modify-write atomically. The atomic utils helper functions introduce just the semi-atomic functions this needs - which at least for word sized variables should be lock free.)

But I intentionally left the implementation of oorb unchanged. Fixing it efficiently would require the atomic utils PR being merged. And fixing it inefficiently is done in tsrb.

[maribu]

Btw: As boths contributions have been split out, there is no need to keep this open.