another cond var fix: requeue count race condition

lock out new waiters during the broadcast. otherwise the wait count
added to the mutex might be lower than the actual number of waiters
moved, and wakeups may be lost.

this issue could also be solved by temporarily setting the mutex
waiter count higher than any possible real count, then relying on the
kernel to tell us how many waiters were requeued, and updating the
counts afterwards. however the logic is more complex, and i don't
really trust the kernel. the solution here is also nice in that it
replaces some atomic cas loops with simple non-atomic ops under lock.

diff --git a/src/thread/pthread_cond_broadcast.c b/src/thread/pthread_cond_broadcast.c
index 9c6a462..848e288 100644 (file)
--- a/src/thread/pthread_cond_broadcast.c
+++ b/src/thread/pthread_cond_broadcast.c
@@ -22,12 +22,8 @@ int pthread_cond_broadcast(pthread_cond_t *c)
        m = c->_c_mutex;
 
        /* Move waiter count to the mutex */
-       for (;;) {
-               int w = c->_c_waiters2;
-               a_fetch_add(&m->_m_waiters, w);
-               if (a_cas(&c->_c_waiters2, w, 0) == w) break;
-               a_fetch_add(&m->_m_waiters, -w);
-       }
+       a_fetch_add(&m->_m_waiters, c->_c_waiters2);
+       c->_c_waiters2 = 0;
 
        /* Perform the futex requeue, waking one waiter unless we know
         * that the calling thread holds the mutex. */

diff --git a/src/thread/pthread_cond_timedwait.c b/src/thread/pthread_cond_timedwait.c
index e3dc814..db2744b 100644 (file)
--- a/src/thread/pthread_cond_timedwait.c
+++ b/src/thread/pthread_cond_timedwait.c
@@ -7,8 +7,6 @@ struct cm {
 
 static void unwait(pthread_cond_t *c, pthread_mutex_t *m)
 {
-       int w;
-
        /* Removing a waiter is non-trivial if we could be using requeue
         * based broadcast signals, due to mutex access issues, etc. */
 
@@ -20,10 +18,8 @@ static void unwait(pthread_cond_t *c, pthread_mutex_t *m)
        while (a_swap(&c->_c_lock, 1))
                __wait(&c->_c_lock, &c->_c_lockwait, 1, 1);
 
-       /* Atomically decrement waiters2 if positive, else mutex waiters. */
-       do w = c->_c_waiters2;
-       while (w && a_cas(&c->_c_waiters2, w, w-1)!=w);
-       if (!w) a_dec(&m->_m_waiters);
+       if (c->_c_waiters2) c->_c_waiters2--;
+       else a_dec(&m->_m_waiters);
 
        a_store(&c->_c_lock, 0);
        if (c->_c_lockwait) __wake(&c->_c_lock, 1, 1);
@@ -46,10 +42,16 @@ int pthread_cond_timedwait(pthread_cond_t *c, pthread_mutex_t *m, const struct t
 
        pthread_testcancel();
 
-       if (c->_c_mutex != (void *)-1) c->_c_mutex = m;
-
        a_inc(&c->_c_waiters);
-       a_inc(&c->_c_waiters2);
+
+       if (c->_c_mutex != (void *)-1) {
+               c->_c_mutex = m;
+               while (a_swap(&c->_c_lock, 1))
+                       __wait(&c->_c_lock, &c->_c_lockwait, 1, 1);
+               c->_c_waiters2++;
+               a_store(&c->_c_lock, 0);
+               if (c->_c_lockwait) __wake(&c->_c_lock, 1, 1);
+       }
 
        seq = c->_c_seq;