correctly handle the degenerate barrier in the pshared case

[musl] / src / thread / pthread_barrier_wait.c

diff --git a/src/thread/pthread_barrier_wait.c b/src/thread/pthread_barrier_wait.c
index 02c252a..a97f6d5 100644 (file)
--- a/src/thread/pthread_barrier_wait.c
+++ b/src/thread/pthread_barrier_wait.c
@@ -1,31 +1,118 @@
 #include "pthread_impl.h"
 
+static int vmlock[2];
+
+void __vm_lock(int inc)
+{
+       for (;;) {
+               int v = vmlock[0];
+               if (inc*v < 0) __wait(vmlock, vmlock+1, v, 1);
+               else if (a_cas(vmlock, v, v+inc)==v) break;
+       }
+}
+
+void __vm_unlock(void)
+{
+       if (vmlock[0]>0) a_dec(vmlock);
+       else a_inc(vmlock);
+       if (vmlock[1]) __wake(vmlock, 1, 1);
+}
+
+static int pshared_barrier_wait(pthread_barrier_t *b)
+{
+       int limit = (b->_b_limit & INT_MAX) + 1;
+       int seq;
+       int ret = 0;
+
+       if (limit==1) return PTHREAD_BARRIER_SERIAL_THREAD;
+
+       while (a_swap(&b->_b_lock, 1))
+               __wait(&b->_b_lock, &b->_b_waiters, 1, 0);
+
+       seq = b->_b_seq;
+
+       if (++b->_b_count == limit) {
+               ret = PTHREAD_BARRIER_SERIAL_THREAD;
+               b->_b_seq++;
+               __wake(&b->_b_seq, -1, 0);
+       } else {
+               a_store(&b->_b_lock, 0);
+               if (b->_b_waiters) __wake(&b->_b_lock, 1, 0);
+               __wait(&b->_b_seq, 0, seq, 0);
+       }
+
+       __vm_lock(+1);
+
+       if (a_fetch_add(&b->_b_count, -1)==1) {
+               b->_b_seq++;
+               __wake(&b->_b_seq, -1, 0);
+               a_store(&b->_b_lock, 0);
+               if (b->_b_waiters) __wake(&b->_b_lock, 1, 0);
+       } else {
+               __wait(&b->_b_seq, 0, seq+1, 0);
+       }
+       
+       __vm_unlock();
+
+       return ret;
+}
+
+struct instance
+{
+       int count;
+       int last;
+       int waiters;
+       int finished;
+};
+
 int pthread_barrier_wait(pthread_barrier_t *b)
 {
-       int cur;
+       int limit = b->_b_limit;
+       struct instance *inst;
 
        /* Trivial case: count was set at 1 */
-       if (!b->__limit) return PTHREAD_BARRIER_SERIAL_THREAD;
+       if (!limit) return PTHREAD_BARRIER_SERIAL_THREAD;
+
+       /* Process-shared barriers require a separate, inefficient wait */
+       if (limit < 0) return pshared_barrier_wait(b);
 
-       /* Wait for anyone still suspended at previous use of barrier */
-       while ((cur=b->__left))
-               __wait(&b->__left, &b->__waiters, cur, 0);
+       /* 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;
 
-       /* If we are the last to reach barrier, reset it and wake others */
-       if (a_fetch_add(&b->__count, 1) == b->__limit) {
-               b->__left = b->__limit;
-               b->__count = 0;
-               __wake(&b->__count, -1, 0);
+       /* First thread to enter the barrier becomes the "instance owner" */
+       if (!inst) {
+               struct instance new_inst = { 0 };
+               int spins = 10000;
+               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,1,0);
                return PTHREAD_BARRIER_SERIAL_THREAD;
        }
 
-       /* Wait for our peers to reach the barrier */
-       while ((cur=b->__count))
-               __wait(&b->__count, 0, cur, 0);
+       /* 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);
+       }
 
-       /* If we're the last to wake up and barrier is awaiting reuse */
-       if (a_fetch_add(&b->__left, -1) == 1 && b->__waiters)
-               __wake(&b->__left, -1, 0);
+       /* 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;
 }