fix ABA race in cond vars, improve them overall

[musl] / src / thread / pthread_cond_signal.c

#include "pthread_impl.h"

int pthread_cond_signal(pthread_cond_t *c)
{
        a_store(&c->_c_block, 0);
        if (c->_c_waiters) __wake(&c->_c_block, 1, 0);
        return 0;
}