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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.
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