commit c2f5813ae0a68f6c6d69e66dac2da6e46b9df034
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Wed Mar 18 17:05:38 2015 +0000

    Make sem_timedwait use FUTEX_CLOCK_REALTIME (bug 18138).
    
    sem_timedwait converts absolute timeouts to relative to pass them to
    the futex syscall.  (Before the recent reimplementation, on x86_64 it
    used FUTEX_CLOCK_REALTIME, but not on other architectures.)
    
    Correctly implementing POSIX requirements, however, requires use of
    FUTEX_CLOCK_REALTIME; passing a relative timeout to the kernel does
    not conform to POSIX.  The POSIX specification for sem_timedwait says
    "The timeout shall be based on the CLOCK_REALTIME clock.".  The POSIX
    specification for clock_settime says "If the value of the
    CLOCK_REALTIME clock is set via clock_settime(), the new value of the
    clock shall be used to determine the time of expiration for absolute
    time services based upon the CLOCK_REALTIME clock. This applies to the
    time at which armed absolute timers expire. If the absolute time
    requested at the invocation of such a time service is before the new
    value of the clock, the time service shall expire immediately as if
    the clock had reached the requested time normally.".  If a relative
    timeout is passed to the kernel, it is interpreted according to the
    CLOCK_MONOTONIC clock, and so fails to meet that POSIX requirement in
    the event of clock changes.
    
    This patch makes sem_timedwait use lll_futex_timed_wait_bitset with
    FUTEX_CLOCK_REALTIME when possible, as done in some other places in
    NPTL.  FUTEX_CLOCK_REALTIME is always available for supported Linux
    kernel versions; unavailability of lll_futex_timed_wait_bitset is only
    an issue for hppa (an issue noted in
    <https://sourceware.org/glibc/wiki/PortStatus>, and fixed by the
    unreviewed
    <https://sourceware.org/ml/libc-alpha/2014-12/msg00655.html> that
    removes the hppa lowlevellock.h completely).
    
    In the FUTEX_CLOCK_REALTIME case, the glibc code still needs to check
    for negative tv_sec and handle that as timeout, because the Linux
    kernel returns EINVAL not ETIMEDOUT for that case, so resulting in
    failures of nptl/tst-abstime and nptl/tst-sem13 in the absence of that
    check.  If we're trying to distinguish between Linux-specific and
    generic-futex NPTL code, I suppose having this in an nptl/ file isn't
    ideal, but there doesn't seem to be any better place at present.
    
    It's not possible to add a testcase for this issue to the testsuite
    because of the requirement to change the system clock as part of a
    test (this is a case where testing would require some form of
    container, with root in that container, and one whose CLOCK_REALTIME
    is isolated from that of the host; I'm not sure what forms of
    containers, short of a full virtual machine, provide that clock
    isolation).
    
    Tested for x86_64.  Also tested for powerpc with the testcase included
    in the bug.
    
        [BZ #18138]
        * nptl/sem_waitcommon.c: Include <kernel-features.h>.
        (futex_abstimed_wait)
        [__ASSUME_FUTEX_CLOCK_REALTIME && lll_futex_timed_wait_bitset]:
        Use lll_futex_timed_wait_bitset with FUTEX_CLOCK_REALTIME instead
        of lll_futex_timed_wait.

Index: glibc-2.17-c758a686/nptl/sysdeps/unix/sysv/linux/sem_waitcommon.c
===================================================================
--- glibc-2.17-c758a686.orig/nptl/sysdeps/unix/sysv/linux/sem_waitcommon.c
+++ glibc-2.17-c758a686/nptl/sysdeps/unix/sysv/linux/sem_waitcommon.c
@@ -17,6 +17,7 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
+#include <kernel-features.h>
 #include <errno.h>
 #include <sysdep.h>
 #include <lowlevellock.h>
@@ -45,6 +46,13 @@ futex_abstimed_wait (unsigned int* futex
     }
   else
     {
+#if (defined __ASSUME_FUTEX_CLOCK_REALTIME	\
+     && defined lll_futex_timed_wait_bitset)
+      /* The Linux kernel returns EINVAL for this, but in userspace
+	 such a value is valid.  */
+      if (abstime->tv_sec < 0)
+	return ETIMEDOUT;
+#else
       struct timeval tv;
       struct timespec rt;
       int sec, nsec;
@@ -68,9 +76,16 @@ futex_abstimed_wait (unsigned int* futex
       /* Do wait.  */
       rt.tv_sec = sec;
       rt.tv_nsec = nsec;
+#endif
       if (cancel)
 	oldtype = __pthread_enable_asynccancel ();
+#if (defined __ASSUME_FUTEX_CLOCK_REALTIME	\
+     && defined lll_futex_timed_wait_bitset)
+      err = lll_futex_timed_wait_bitset (futex, expected, abstime,
+					 FUTEX_CLOCK_REALTIME, private);
+#else
       err = lll_futex_timed_wait (futex, expected, &rt, private);
+#endif
       if (cancel)
 	__pthread_disable_asynccancel (oldtype);
     }