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, 7 insertions, 2 deletions

diff --git a/nptl/sem_open.c b/nptl/sem_open.c
index b5a5de4eeb..bfd2dea94e 100644
--- a/nptl/sem_open.c
+++ b/nptl/sem_open.c
@@ -193,9 +193,14 @@ sem_open (const char *name, int oflag, ...)
 	struct new_sem newsem;
       } sem;
 
-      sem.newsem.value = value;
-      sem.newsem.private = 0;
+#if __HAVE_64B_ATOMICS
+      sem.newsem.data = value;
+#else
+      sem.newsem.value = value << SEM_VALUE_SHIFT;
       sem.newsem.nwaiters = 0;
+#endif
+      /* This always is a shared semaphore.  */
+      sem.newsem.private = LLL_SHARED;
 
       /* Initialize the remaining bytes as well.  */
       memset ((char *) &sem.initsem + sizeof (struct new_sem), '\0',