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Make sem_timedwait use FUTEX_CLOCK_REALTIME (bug 18138).

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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.
This commit is contained in:
Joseph Myers 2015-03-18 17:05:38 +00:00
parent 3382c079da
commit c2f5813ae0
3 changed files with 25 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
ChangeLog
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@ -1,3 +1,12 @@
2015-03-18 Joseph Myers <joseph@codesourcery.com>
[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.
2015-03-18 Brad Hubbard <bhubbard@redhat.com>
[BZ #17542]

2
NEWS
View File

@ -15,7 +15,7 @@ Version 2.22
17932, 17944, 17949, 17964, 17965, 17967, 17969, 17978, 17987, 17991,
17996, 17998, 17999, 18019, 18020, 18029, 18030, 18032, 18036, 18038,
18039, 18042, 18043, 18046, 18047, 18068, 18080, 18093, 18104, 18110,
18111, 18128.
18111, 18128, 18138.
* Character encoding and ctype tables were updated to Unicode 7.0.0, using
new generator scripts contributed by Pravin Satpute and Mike FABIAN (Red

15
nptl/sem_waitcommon.c
View File

@ -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, unsigned int expected,
}
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, unsigned int expected,
/* 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);
}