Redesigned how cancellation unblocks a thread from internal cancellation points (sem_wait, pthread_join, pthread_cond_{wait,timedwait}). Cancellation won't eat a signal in any of these functions (*required* by POSIX and Single Unix Spec!).

2000-01-03  Kaz Kylheku  <kaz@ashi.footprints.net>

	Redesigned how cancellation unblocks a thread from internal
	cancellation points (sem_wait, pthread_join,
	pthread_cond_{wait,timedwait}).
	Cancellation won't eat a signal in any of these functions
	(*required* by POSIX and Single Unix Spec!).
	* condvar.c: spontaneous wakeup on pthread_cond_timedwait won't eat a
	simultaneous condition variable signal (not required by POSIX
	or Single Unix Spec, but nice).
	* spinlock.c: __pthread_lock queues back any received restarts
	that don't belong to it instead of assuming ownership of lock
	upon any restart; fastlock can no longer be acquired by two threads
	simultaneously.
	* restart.h: restarts queue even on kernels that don't have
	queued real time signals (2.0, early 2.1), thanks to atomic counter,
	avoiding a rare race condition in pthread_cond_timedwait.

1 files changed, 45 insertions, 9 deletions

diff --git a/linuxthreads/semaphore.c b/linuxthreads/semaphore.c
index cfad97aced..0297b3a1e3 100644
--- a/linuxthreads/semaphore.c
+++ b/linuxthreads/semaphore.c
@@ -38,9 +38,31 @@ int __new_sem_init(sem_t *sem, int pshared, unsigned int value)
   return 0;
 }
 
+/* Function called by pthread_cancel to remove the thread from
+   waiting inside __new_sem_wait. */
+
+static int new_sem_extricate_func(void *obj, pthread_descr th)
+{
+  volatile pthread_descr self = thread_self();
+  sem_t *sem = obj;
+  int did_remove = 0;
+
+  __pthread_lock((struct _pthread_fastlock *) &sem->__sem_lock, self);
+  did_remove = remove_from_queue(&sem->__sem_waiting, th);
+  __pthread_unlock((struct _pthread_fastlock *) &sem->__sem_lock);
+
+  return did_remove;
+}
+
 int __new_sem_wait(sem_t * sem)
 {
   volatile pthread_descr self = thread_self();
+  pthread_extricate_if extr;
+  int already_canceled = 0;
+
+  /* Set up extrication interface */
+  extr.pu_object = sem;
+  extr.pu_extricate_func = new_sem_extricate_func;
 
   __pthread_lock((struct _pthread_fastlock *) &sem->__sem_lock, self);
   if (sem->__sem_value > 0) {
@@ -48,17 +70,31 @@ int __new_sem_wait(sem_t * sem)
     __pthread_unlock((struct _pthread_fastlock *) &sem->__sem_lock);
     return 0;
   }
-  enqueue(&sem->__sem_waiting, self);
-  /* Wait for sem_post or cancellation */
+  /* Register extrication interface */
+  __pthread_set_own_extricate_if(self, &extr); 
+  /* Enqueue only if not already cancelled. */
+  if (!(THREAD_GETMEM(self, p_canceled)
+      && THREAD_GETMEM(self, p_cancelstate) == PTHREAD_CANCEL_ENABLE))
+    enqueue(&sem->__sem_waiting, self);
+  else
+    already_canceled = 1;
   __pthread_unlock((struct _pthread_fastlock *) &sem->__sem_lock);
-  suspend_with_cancellation(self);
-  /* This is a cancellation point */
-  if (THREAD_GETMEM(self, p_canceled)
+
+  if (already_canceled) {
+    __pthread_set_own_extricate_if(self, 0); 
+    pthread_exit(PTHREAD_CANCELED);
+  } 
+
+  /* Wait for sem_post or cancellation, or fall through if already canceled */
+  suspend(self);
+  __pthread_set_own_extricate_if(self, 0); 
+
+  /* Terminate only if the wakeup came from cancellation. */
+  /* Otherwise ignore cancellation because we got the semaphore. */
+
+  if (THREAD_GETMEM(self, p_woken_by_cancel)
       && THREAD_GETMEM(self, p_cancelstate) == PTHREAD_CANCEL_ENABLE) {
-    /* Remove ourselves from the waiting list if we're still on it */
-    __pthread_lock((struct _pthread_fastlock *) &sem->__sem_lock, self);
-    remove_from_queue(&sem->__sem_waiting, self);
-    __pthread_unlock((struct _pthread_fastlock *) &sem->__sem_lock);
+    THREAD_SETMEM(self, p_woken_by_cancel, 0);
     pthread_exit(PTHREAD_CANCELED);
   }
   /* We got the semaphore */