Fix semaphore destruction (bug 12674).

This commit fixes semaphore destruction by either using 64b atomic
operations (where available), or by using two separate fields when only
32b atomic operations are available.  In the latter case, we keep a
conservative estimate of whether there are any waiting threads in one
bit of the field that counts the number of available tokens, thus
allowing sem_post to atomically both add a token and determine whether
it needs to call futex_wake.

See:
https://sourceware.org/ml/libc-alpha/2014-12/msg00155.html

1 files changed, 0 insertions, 46 deletions

diff --git a/nptl/DESIGN-sem.txt b/nptl/DESIGN-sem.txt
deleted file mode 100644
index 17eb0c1..0000000
--- a/nptl/DESIGN-sem.txt
+++ /dev/null
@@ -1,46 +0,0 @@
-Semaphores pseudocode
-==============================
-
-       int sem_wait(sem_t * sem);
-       int sem_trywait(sem_t * sem);
-       int sem_post(sem_t * sem);
-       int sem_getvalue(sem_t * sem, int * sval);
-
-struct sem_t {
-
-   unsigned int count;
-         - current semaphore count, also used as a futex
-}
-
-sem_wait(sem_t *sem)
-{
-  for (;;) {
-
-    if (atomic_decrement_if_positive(sem->count))
-      break;
-
-    futex_wait(&sem->count, 0)
-  }
-}
-
-sem_post(sem_t *sem)
-{
-  n = atomic_increment(sem->count);
-  // Pass the new value of sem->count
-  futex_wake(&sem->count, n + 1);
-}
-
-sem_trywait(sem_t *sem)
-{
-  if (atomic_decrement_if_positive(sem->count)) {
-    return 0;
-  } else {
-    return EAGAIN;
-  }
-}
-
-sem_getvalue(sem_t *sem, int *sval)
-{
-  *sval = sem->count;
-  read_barrier();
-}