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, 23 insertions, 12 deletions

diff --git a/nptl/sem_init.c b/nptl/sem_init.c
index 5350f16a19..575b661f62 100644
--- a/nptl/sem_init.c
+++ b/nptl/sem_init.c
@@ -18,17 +18,29 @@
 
 #include <errno.h>
 #include <semaphore.h>
-#include <lowlevellock.h>
 #include <shlib-compat.h>
 #include "semaphoreP.h"
 #include <kernel-features.h>
 
+/* Returns FUTEX_PRIVATE if pshared is zero and private futexes are supported;
+   returns FUTEX_SHARED otherwise.
+   TODO Remove when cleaning up the futex API throughout glibc.  */
+static __always_inline int
+futex_private_if_supported (int pshared)
+{
+  if (pshared != 0)
+    return LLL_SHARED;
+#ifdef __ASSUME_PRIVATE_FUTEX
+  return LLL_PRIVATE;
+#else
+  return THREAD_GETMEM (THREAD_SELF, header.private_futex)
+      ^ FUTEX_PRIVATE_FLAG;
+#endif
+}
+
 
 int
-__new_sem_init (sem, pshared, value)
-     sem_t *sem;
-     int pshared;
-     unsigned int value;
+__new_sem_init (sem_t *sem, int pshared, unsigned int value)
 {
   /* Parameter sanity check.  */
   if (__glibc_unlikely (value > SEM_VALUE_MAX))
@@ -40,16 +52,15 @@ __new_sem_init (sem, pshared, value)
   /* Map to the internal type.  */
   struct new_sem *isem = (struct new_sem *) sem;
 
-  /* Use the values the user provided.  */
-  isem->value = value;
-#ifdef __ASSUME_PRIVATE_FUTEX
-  isem->private = pshared ? 0 : FUTEX_PRIVATE_FLAG;
+  /* Use the values the caller provided.  */
+#if __HAVE_64B_ATOMICS
+  isem->data = value;
 #else
-  isem->private = pshared ? 0 : THREAD_GETMEM (THREAD_SELF,
-					       header.private_futex);
+  isem->value = value << SEM_VALUE_SHIFT;
+  isem->nwaiters = 0;
 #endif
 
-  isem->nwaiters = 0;
+  isem->private = futex_private_if_supported (pshared);
 
   return 0;
 }