Add a "fork" of musl as //fusl.

fusl/src/thread/pthread_barrier_wait.c

+#include "pthread_impl.h"
+
+static int pshared_barrier_wait(pthread_barrier_t *b)
+{
+        int limit = (b->_b_limit & INT_MAX) + 1;
+        int ret = 0;
+        int v, w;
+
+        if (limit==1) return PTHREAD_BARRIER_SERIAL_THREAD;
+
+        while ((v=a_cas(&b->_b_lock, 0, limit)))
+                __wait(&b->_b_lock, &b->_b_waiters, v, 0);
+
+        /* Wait for <limit> threads to get to the barrier */
+        if (++b->_b_count == limit) {
+                a_store(&b->_b_count, 0);
+                ret = PTHREAD_BARRIER_SERIAL_THREAD;
+                if (b->_b_waiters2) __wake(&b->_b_count, -1, 0);
+        } else {
+                a_store(&b->_b_lock, 0);
+                if (b->_b_waiters) __wake(&b->_b_lock, 1, 0);
+                while ((v=b->_b_count)>0)
+                        __wait(&b->_b_count, &b->_b_waiters2, v, 0);
+        }
+
+        __vm_lock();
+
+        /* Ensure all threads have a vm lock before proceeding */
+        if (a_fetch_add(&b->_b_count, -1)==1-limit) {
+                a_store(&b->_b_count, 0);
+                if (b->_b_waiters2) __wake(&b->_b_count, -1, 0);
+        } else {
+                while ((v=b->_b_count))
+                        __wait(&b->_b_count, &b->_b_waiters2, v, 0);
+        }
+        
+        /* Perform a recursive unlock suitable for self-sync'd destruction */
+        do {
+                v = b->_b_lock;
+                w = b->_b_waiters;
+        } while (a_cas(&b->_b_lock, v, v==INT_MIN+1 ? 0 : v-1) != v);
+
+        /* Wake a thread waiting to reuse or destroy the barrier */
+        if (v==INT_MIN+1 || (v==1 && w))
+                __wake(&b->_b_lock, 1, 0);
+
+        __vm_unlock();
+
+        return ret;
+}
+
+struct instance
+{
+        volatile int count;
+        volatile int last;
+        volatile int waiters;
+        volatile int finished;
+};
+
+int pthread_barrier_wait(pthread_barrier_t *b)
+{
+        int limit = b->_b_limit;
+        struct instance *inst;
+
+        /* Trivial case: count was set at 1 */
+        if (!limit) return PTHREAD_BARRIER_SERIAL_THREAD;
+
+        /* Process-shared barriers require a separate, inefficient wait */
+        if (limit < 0) return pshared_barrier_wait(b);
+
+        /* Otherwise we need a lock on the barrier object */
+        while (a_swap(&b->_b_lock, 1))
+                __wait(&b->_b_lock, &b->_b_waiters, 1, 1);
+        inst = b->_b_inst;
+
+        /* First thread to enter the barrier becomes the "instance owner" */
+        if (!inst) {
+                struct instance new_inst = { 0 };
+                int spins = 200;
+                b->_b_inst = inst = &new_inst;
+                a_store(&b->_b_lock, 0);
+                if (b->_b_waiters) __wake(&b->_b_lock, 1, 1);
+                while (spins-- && !inst->finished)
+                        a_spin();
+                a_inc(&inst->finished);
+                while (inst->finished == 1)
+                        __syscall(SYS_futex,&inst->finished,FUTEX_WAIT|128,1,0) != -ENOSYS
+                        || __syscall(SYS_futex,&inst->finished,FUTEX_WAIT,1,0);
+                return PTHREAD_BARRIER_SERIAL_THREAD;
+        }
+
+        /* Last thread to enter the barrier wakes all non-instance-owners */
+        if (++inst->count == limit) {
+                b->_b_inst = 0;
+                a_store(&b->_b_lock, 0);
+                if (b->_b_waiters) __wake(&b->_b_lock, 1, 1);
+                a_store(&inst->last, 1);
+                if (inst->waiters)
+                        __wake(&inst->last, -1, 1);
+        } else {
+                a_store(&b->_b_lock, 0);
+                if (b->_b_waiters) __wake(&b->_b_lock, 1, 1);
+                __wait(&inst->last, &inst->waiters, 0, 1);
+        }
+
+        /* Last thread to exit the barrier wakes the instance owner */
+        if (a_fetch_add(&inst->count,-1)==1 && a_fetch_add(&inst->finished,1))
+                __wake(&inst->finished, 1, 1);
+
+        return 0;
+}