fix extremely rare but dangerous race condition in robust mutexes

[musl] / src / thread / pthread_mutex_unlock.c

diff --git a/src/thread/pthread_mutex_unlock.c b/src/thread/pthread_mutex_unlock.c
index 05baec1..5fc0f4e 100644 (file)
--- a/src/thread/pthread_mutex_unlock.c
+++ b/src/thread/pthread_mutex_unlock.c
@@ -1,30 +1,36 @@
 #include "pthread_impl.h"
 
+void __vm_lock_impl(int);
+void __vm_unlock_impl(void);
+
 int pthread_mutex_unlock(pthread_mutex_t *m)
 {
        pthread_t self;
        int waiters = m->_m_waiters;
        int cont;
+       int robust = 0;
 
        if (m->_m_type != PTHREAD_MUTEX_NORMAL) {
                if (!m->_m_lock)
                        return EPERM;
-               self = __pthread_self();
+               self = pthread_self();
                if ((m->_m_lock&0x1fffffff) != self->tid)
                        return EPERM;
                if ((m->_m_type&3) == PTHREAD_MUTEX_RECURSIVE && m->_m_count)
                        return m->_m_count--, 0;
                if (m->_m_type >= 4) {
+                       robust = 1;
                        self->robust_list.pending = &m->_m_next;
                        *(void **)m->_m_prev = m->_m_next;
                        if (m->_m_next) ((void **)m->_m_next)[-1] = m->_m_prev;
-                       cont = a_swap(&m->_m_lock, 0);
-                       self->robust_list.pending = 0;
-                       goto wake;
+                       __vm_lock_impl(+1);
                }
        }
        cont = a_swap(&m->_m_lock, 0);
-wake:
+       if (robust) {
+               self->robust_list.pending = 0;
+               __vm_unlock_impl();
+       }
        if (waiters || cont<0)
                __wake(&m->_m_lock, 1, 0);
        return 0;