Do not violate mutex destruction requirements.

POSIX and C++11 require that a thread can destroy a mutex if no other
thread owns the mutex, is blocked on the mutex, or will try to acquire
it in the future.  After destroying the mutex, it can reuse or unmap the
underlying memory.  Thus, we must not access a mutex' memory after
releasing it.  Currently, we can load the private flag after releasing
the mutex, which is fixed by this patch.
See https://sourceware.org/bugzilla/show_bug.cgi?id=13690 for more
background.

We need to call futex_wake on the lock after releasing it, however.  This
is by design, and can lead to spurious wake-ups on unrelated futex words
(e.g., when the mutex memory is reused for another mutex).  This behavior
is documented in the glibc-internal futex API and in recent drafts of the
Linux kernel's futex documentation (see the draft_futex branch of
git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git).

1 files changed, 7 insertions, 2 deletions

diff --git a/sysdeps/unix/sysv/linux/lowlevellock-futex.h b/sysdeps/unix/sysv/linux/lowlevellock-futex.h
index 59f6627bdb..40825f0306 100644
--- a/sysdeps/unix/sysv/linux/lowlevellock-futex.h
+++ b/sysdeps/unix/sysv/linux/lowlevellock-futex.h
@@ -54,8 +54,13 @@
 #if IS_IN (libc) || IS_IN (rtld)
 /* In libc.so or ld.so all futexes are private.  */
 # ifdef __ASSUME_PRIVATE_FUTEX
-#  define __lll_private_flag(fl, private) \
-  ((fl) | FUTEX_PRIVATE_FLAG)
+#  define __lll_private_flag(fl, private)			\
+  ({								\
+    /* Prevent warnings in callers of this macro.  */		\
+    int __lll_private_flag_priv __attribute__ ((unused));	\
+    __lll_private_flag_priv = (private);			\
+    ((fl) | FUTEX_PRIVATE_FLAG);				\
+  })
 # else
 #  define __lll_private_flag(fl, private) \
   ((fl) | THREAD_GETMEM (THREAD_SELF, header.private_futex))