Re: igc, macOS avoiding signals

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

Gerd Möllmann <gerd.moellmann@gmail.com> writes:

> Hm. Have you perhaps looked at a pthread implementation, what such a
> mutex actually is on Linux?

Unless I got lost in Apple's sources, trylock on macOS is the below.
I find it hard to tell if a signal here while being called from a signal
would do damage.

int
pthread_mutex_trylock(pthread_mutex_t *mutex)
{
	return _pthread_mutex_lock(mutex, true);
}

static inline int
_pthread_mutex_lock(pthread_mutex_t *mutex, bool trylock)
{
	if (os_unlikely(!_pthread_mutex_check_signature_fast(mutex))) {
		return _pthread_mutex_lock_init_slow(mutex, trylock);
	}

	if (os_unlikely(_pthread_mutex_is_fairshare(mutex))) {
		return _pthread_mutex_fairshare_lock(mutex, trylock);
	}

	if (os_unlikely(_pthread_mutex_uses_ulock(mutex))) {
		return _pthread_mutex_ulock_lock(mutex, trylock);
	}

#if ENABLE_USERSPACE_TRACE
	return _pthread_mutex_firstfit_lock_slow(mutex, trylock);
#elif PLOCKSTAT
	if (PLOCKSTAT_MUTEX_ACQUIRE_ENABLED() || PLOCKSTAT_MUTEX_ERROR_ENABLED()) {
		return _pthread_mutex_firstfit_lock_slow(mutex, trylock);
	}
#endif

	return _pthread_mutex_firstfit_lock(mutex, trylock);
}

static inline int
_pthread_mutex_firstfit_lock(pthread_mutex_t *mutex, bool trylock)
{
	/*
	 * This is the first-fit fast path. The fairshare fast-ish path is in
	 * _pthread_mutex_fairshare_lock()
	 */
	uint64_t *tidaddr;
	MUTEX_GETTID_ADDR(mutex, &tidaddr);
	uint64_t selfid = _pthread_threadid_self_np_direct();

	mutex_seq *seqaddr;
	MUTEX_GETSEQ_ADDR(mutex, &seqaddr);

	mutex_seq oldseq, newseq;
	mutex_seq_load(seqaddr, &oldseq);

	if (os_unlikely(!trylock && (oldseq.lgenval & PTH_RWL_EBIT))) {
		return _pthread_mutex_firstfit_lock_slow(mutex, trylock);
	}

	bool gotlock;
	do {
		newseq = oldseq;
		gotlock = is_rwl_ebit_clear(oldseq.lgenval);

		if (trylock && !gotlock) {
#if __LP64__
			// The sequence load is atomic, so we can bail here without writing
			// it and avoid some unnecessary coherence traffic - rdar://57259033
			os_atomic_thread_fence(acquire);
			return EBUSY;
#else
			// A trylock on a held lock will fail immediately. But since
			// we did not load the sequence words atomically, perform a
			// no-op CAS64 to ensure that nobody has unlocked concurrently.
#endif
		} else if (os_likely(gotlock)) {
			// In first-fit, getting the lock simply adds the E-bit
			newseq.lgenval |= PTH_RWL_EBIT;
		} else {
			return _pthread_mutex_firstfit_lock_slow(mutex, trylock);
		}
	} while (os_unlikely(!mutex_seq_atomic_cmpxchgv(seqaddr, &oldseq, &newseq,
			acquire)));

	if (os_likely(gotlock)) {
		os_atomic_store_wide(tidaddr, selfid, relaxed);
		return 0;
	} else if (trylock) {
		return EBUSY;
	} else {
		__builtin_trap();
	}
}