mailing list of musl libc

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 #include "pthread_impl.h"
#include <threads.h>

int mtx_trylock(mtx_t *m)
{
	int tid, old, own, ret = 0;
	pthread_t self;

	if (m->_m_type == PTHREAD_MUTEX_NORMAL) {
		return a_cas(&m->_m_lock, 0, thrd_busy) & thrd_busy;
	} else {
       		self = __pthread_self();
       		tid = self->tid;
		old = m->_m_lock;
		own = old & 0x7fffffff;
		if (own == tid) {
			if ((unsigned)m->_m_count >= INT_MAX) return thrd_error;
			else m->_m_count++;
		} else {
			if ((own && !(own & 0x40000000)) || a_cas(&m->_m_lock, old, tid)!=old)
				return thrd_busy;
		}
	}
	return thrd_success;
}