completely new barrier implementation, addressing major correctness issues

[musl] / src / thread / pthread_barrier_wait.c

diff --git a/src/thread/pthread_barrier_wait.c b/src/thread/pthread_barrier_wait.c
index 7bfadb9..1e638d9 100644 (file)
--- a/src/thread/pthread_barrier_wait.c
+++ b/src/thread/pthread_barrier_wait.c
@@ -1,31 +1,59 @@
 #include "pthread_impl.h"
 
+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->_b_limit) return PTHREAD_BARRIER_SERIAL_THREAD;
+       if (!limit) return PTHREAD_BARRIER_SERIAL_THREAD;
 
-       /* Wait for anyone still suspended at previous use of barrier */
-       while ((cur=b->_b_left))
-               __wait(&b->_b_left, &b->_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, 0);
+       inst = b->_b_inst;
 
-       /* If we are the last to reach barrier, reset it and wake others */
-       if (a_fetch_add(&b->_b_count, 1) == b->_b_limit) {
-               b->_b_left = b->_b_limit;
-               b->_b_count = 0;
-               __wake(&b->_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, 0);
+               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->_b_count))
-               __wait(&b->_b_count, 0, cur, 0);
+       /* Last thread to enter the barrier wakes all non-instance-owners */
+       if (++inst->count == limit) {
+a_spin(); a_spin();
+               b->_b_inst = 0;
+               a_store(&b->_b_lock, 0);
+               if (b->_b_waiters) __wake(&b->_b_lock, 1, 0);
+               a_store(&inst->last, 1);
+               if (inst->waiters)
+                       __wake(&inst->last, -1, 0);
+       } else {
+               a_store(&b->_b_lock, 0);
+               if (b->_b_waiters) __wake(&b->_b_lock, 1, 0);
+               __wait(&inst->last, &inst->waiters, 0, 0);
+       }
 
-       /* If we're the last to wake up and barrier is awaiting reuse */
-       if (a_fetch_add(&b->_b_left, -1) == 1 && b->_b_waiters)
-               __wake(&b->_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, 0);
 
        return 0;
 }