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, 20 insertions, 6 deletions

diff --git a/nptl/sem_getvalue.c b/nptl/sem_getvalue.c
index c3c91e1f33..1432cc795e 100644
--- a/nptl/sem_getvalue.c
+++ b/nptl/sem_getvalue.c
@@ -19,23 +19,37 @@
 #include <semaphore.h>
 #include <shlib-compat.h>
 #include "semaphoreP.h"
+#include <atomic.h>
 
 
 int
-__new_sem_getvalue (sem, sval)
-     sem_t *sem;
-     int *sval;
+__new_sem_getvalue (sem_t *sem, int *sval)
 {
   struct new_sem *isem = (struct new_sem *) sem;
 
   /* XXX Check for valid SEM parameter.  */
-
-  *sval = isem->value;
+  /* FIXME This uses relaxed MO, even though POSIX specifies that this function
+     should be linearizable.  However, its debatable whether linearizability
+     is the right requirement.  We need to follow up with POSIX and, if
+     necessary, use a stronger MO here and elsewhere (e.g., potentially
+     release MO in all places where we consume a token).  */
+
+#if __HAVE_64B_ATOMICS
+  *sval = atomic_load_relaxed (&isem->data) & SEM_VALUE_MASK;
+#else
+  *sval = atomic_load_relaxed (&isem->value) >> SEM_VALUE_SHIFT;
+#endif
 
   return 0;
 }
 versioned_symbol (libpthread, __new_sem_getvalue, sem_getvalue, GLIBC_2_1);
 #if SHLIB_COMPAT(libpthread, GLIBC_2_0, GLIBC_2_1)
-strong_alias (__new_sem_getvalue, __old_sem_getvalue)
+int
+__old_sem_getvalue (sem_t *sem, int *sval)
+{
+  struct old_sem *isem = (struct old_sem *) sem;
+  *sval = isem->value;
+  return 0;
+}
 compat_symbol (libpthread, __old_sem_getvalue, sem_getvalue, GLIBC_2_0);
 #endif