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);

     }