mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-23 17:53:37 +08:00
ed19993b5b
This is a new implementation for condition variables, required after http://austingroupbugs.net/view.php?id=609 to fix bug 13165. In essence, we need to be stricter in which waiters a signal or broadcast is required to wake up; this couldn't be solved using the old algorithm. ISO C++ made a similar clarification, so this also fixes a bug in current libstdc++, for example. We can't use the old algorithm anymore because futexes do not guarantee to wake in FIFO order. Thus, when we wake, we can't simply let any waiter grab a signal, but we need to ensure that one of the waiters happening before the signal is woken up. This is something the previous algorithm violated (see bug 13165). There's another issue specific to condvars: ABA issues on the underlying futexes. Unlike mutexes that have just three states, or semaphores that have no tokens or a limited number of them, the state of a condvar is the *order* of the waiters. A waiter on a semaphore can grab a token whenever one is available; a condvar waiter must only consume a signal if it is eligible to do so as determined by the relative order of the waiter and the signal. Therefore, this new algorithm maintains two groups of waiters: Those eligible to consume signals (G1), and those that have to wait until previous waiters have consumed signals (G2). Once G1 is empty, G2 becomes the new G1. 64b counters are used to avoid ABA issues. This condvar doesn't yet use a requeue optimization (ie, on a broadcast, waking just one thread and requeueing all others on the futex of the mutex supplied by the program). I don't think doing the requeue is necessarily the right approach (but I haven't done real measurements yet): * If a program expects to wake many threads at the same time and make that scalable, a condvar isn't great anyway because of how it requires waiters to operate mutually exclusive (due to the mutex usage). Thus, a thundering herd problem is a scalability problem with or without the optimization. Using something like a semaphore might be more appropriate in such a case. * The scalability problem is actually at the mutex side; the condvar could help (and it tries to with the requeue optimization), but it should be the mutex who decides how that is done, and whether it is done at all. * Forcing all but one waiter into the kernel-side wait queue of the mutex prevents/avoids the use of lock elision on the mutex. Thus, it prevents the only cure against the underlying scalability problem inherent to condvars. * If condvars use short critical sections (ie, hold the mutex just to check a binary flag or such), which they should do ideally, then forcing all those waiter to proceed serially with kernel-based hand-off (ie, futex ops in the mutex' contended state, via the futex wait queues) will be less efficient than just letting a scalable mutex implementation take care of it. Our current mutex impl doesn't employ spinning at all, but if critical sections are short, spinning can be much better. * Doing the requeue stuff requires all waiters to always drive the mutex into the contended state. This leads to each waiter having to call futex_wake after lock release, even if this wouldn't be necessary. [BZ #13165] * nptl/pthread_cond_broadcast.c (__pthread_cond_broadcast): Rewrite to use new algorithm. * nptl/pthread_cond_destroy.c (__pthread_cond_destroy): Likewise. * nptl/pthread_cond_init.c (__pthread_cond_init): Likewise. * nptl/pthread_cond_signal.c (__pthread_cond_signal): Likewise. * nptl/pthread_cond_wait.c (__pthread_cond_wait): Likewise. (__pthread_cond_timedwait): Move here from pthread_cond_timedwait.c. (__condvar_confirm_wakeup, __condvar_cancel_waiting, __condvar_cleanup_waiting, __condvar_dec_grefs, __pthread_cond_wait_common): New. (__condvar_cleanup): Remove. * npt/pthread_condattr_getclock.c (pthread_condattr_getclock): Adapt. * npt/pthread_condattr_setclock.c (pthread_condattr_setclock): Likewise. * npt/pthread_condattr_getpshared.c (pthread_condattr_getpshared): Likewise. * npt/pthread_condattr_init.c (pthread_condattr_init): Likewise. * nptl/tst-cond1.c: Add comment. * nptl/tst-cond20.c (do_test): Adapt. * nptl/tst-cond22.c (do_test): Likewise. * sysdeps/aarch64/nptl/bits/pthreadtypes.h (pthread_cond_t): Adapt structure. * sysdeps/arm/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/ia64/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/m68k/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/microblaze/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/mips/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/nios2/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/s390/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/sh/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/tile/nptl/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/unix/sysv/linux/alpha/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/unix/sysv/linux/powerpc/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/x86/bits/pthreadtypes.h (pthread_cond_t): Likewise. * sysdeps/nptl/internaltypes.h (COND_NWAITERS_SHIFT): Remove. (COND_CLOCK_BITS): Adapt. * sysdeps/nptl/pthread.h (PTHREAD_COND_INITIALIZER): Adapt. * nptl/pthreadP.h (__PTHREAD_COND_CLOCK_MONOTONIC_MASK, __PTHREAD_COND_SHARED_MASK): New. * nptl/nptl-printers.py (CLOCK_IDS): Remove. (ConditionVariablePrinter, ConditionVariableAttributesPrinter): Adapt. * nptl/nptl_lock_constants.pysym: Adapt. * nptl/test-cond-printers.py: Adapt. * sysdeps/unix/sysv/linux/hppa/internaltypes.h (cond_compat_clear, cond_compat_check_and_clear): Adapt. * sysdeps/unix/sysv/linux/hppa/pthread_cond_timedwait.c: Remove file ... * sysdeps/unix/sysv/linux/hppa/pthread_cond_wait.c (__pthread_cond_timedwait): ... and move here. * nptl/DESIGN-condvar.txt: Remove file. * nptl/lowlevelcond.sym: Likewise. * nptl/pthread_cond_timedwait.c: Likewise. * sysdeps/unix/sysv/linux/i386/i486/pthread_cond_broadcast.S: Likewise. * sysdeps/unix/sysv/linux/i386/i486/pthread_cond_signal.S: Likewise. * sysdeps/unix/sysv/linux/i386/i486/pthread_cond_timedwait.S: Likewise. * sysdeps/unix/sysv/linux/i386/i486/pthread_cond_wait.S: Likewise. * sysdeps/unix/sysv/linux/i386/i586/pthread_cond_broadcast.S: Likewise. * sysdeps/unix/sysv/linux/i386/i586/pthread_cond_signal.S: Likewise. * sysdeps/unix/sysv/linux/i386/i586/pthread_cond_timedwait.S: Likewise. * sysdeps/unix/sysv/linux/i386/i586/pthread_cond_wait.S: Likewise. * sysdeps/unix/sysv/linux/i386/i686/pthread_cond_broadcast.S: Likewise. * sysdeps/unix/sysv/linux/i386/i686/pthread_cond_signal.S: Likewise. * sysdeps/unix/sysv/linux/i386/i686/pthread_cond_timedwait.S: Likewise. * sysdeps/unix/sysv/linux/i386/i686/pthread_cond_wait.S: Likewise. * sysdeps/unix/sysv/linux/x86_64/pthread_cond_broadcast.S: Likewise. * sysdeps/unix/sysv/linux/x86_64/pthread_cond_signal.S: Likewise. * sysdeps/unix/sysv/linux/x86_64/pthread_cond_timedwait.S: Likewise. * sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S: Likewise. |
||
---|---|---|
argp | ||
assert | ||
benchtests | ||
bits | ||
catgets | ||
conform | ||
crypt | ||
csu | ||
ctype | ||
debug | ||
dirent | ||
dlfcn | ||
elf | ||
gmon | ||
gnulib | ||
grp | ||
gshadow | ||
hesiod | ||
hurd | ||
iconv | ||
iconvdata | ||
include | ||
inet | ||
intl | ||
io | ||
libidn | ||
libio | ||
locale | ||
localedata | ||
login | ||
mach | ||
malloc | ||
manual | ||
math | ||
mathvec | ||
misc | ||
nis | ||
nptl | ||
nptl_db | ||
nscd | ||
nss | ||
po | ||
posix | ||
pwd | ||
resolv | ||
resource | ||
rt | ||
scripts | ||
setjmp | ||
shadow | ||
signal | ||
socket | ||
soft-fp | ||
stdio-common | ||
stdlib | ||
streams | ||
string | ||
sunrpc | ||
support | ||
sysdeps | ||
sysvipc | ||
termios | ||
time | ||
timezone | ||
wcsmbs | ||
wctype | ||
.gitattributes | ||
.gitignore | ||
abi-tags | ||
aclocal.m4 | ||
BUGS | ||
ChangeLog | ||
ChangeLog.1 | ||
ChangeLog.2 | ||
ChangeLog.3 | ||
ChangeLog.4 | ||
ChangeLog.5 | ||
ChangeLog.6 | ||
ChangeLog.7 | ||
ChangeLog.8 | ||
ChangeLog.9 | ||
ChangeLog.10 | ||
ChangeLog.11 | ||
ChangeLog.12 | ||
ChangeLog.13 | ||
ChangeLog.14 | ||
ChangeLog.15 | ||
ChangeLog.16 | ||
ChangeLog.17 | ||
ChangeLog.old-ports | ||
ChangeLog.old-ports-aarch64 | ||
ChangeLog.old-ports-aix | ||
ChangeLog.old-ports-alpha | ||
ChangeLog.old-ports-am33 | ||
ChangeLog.old-ports-arm | ||
ChangeLog.old-ports-cris | ||
ChangeLog.old-ports-hppa | ||
ChangeLog.old-ports-ia64 | ||
ChangeLog.old-ports-linux-generic | ||
ChangeLog.old-ports-m68k | ||
ChangeLog.old-ports-microblaze | ||
ChangeLog.old-ports-mips | ||
ChangeLog.old-ports-powerpc | ||
ChangeLog.old-ports-tile | ||
config.h.in | ||
config.make.in | ||
configure | ||
configure.ac | ||
CONFORMANCE | ||
COPYING | ||
COPYING.LIB | ||
cppflags-iterator.mk | ||
extra-lib.mk | ||
extra-modules.mk | ||
gen-locales.mk | ||
INSTALL | ||
libc-abis | ||
LICENSES | ||
Makeconfig | ||
Makefile | ||
Makefile.in | ||
Makerules | ||
NAMESPACE | ||
NEWS | ||
o-iterator.mk | ||
README | ||
README.pretty-printers | ||
Rules | ||
shlib-versions | ||
test-skeleton.c | ||
version.h | ||
WUR-REPORT |
This directory contains the sources of the GNU C Library. See the file "version.h" for what release version you have. The GNU C Library is the standard system C library for all GNU systems, and is an important part of what makes up a GNU system. It provides the system API for all programs written in C and C-compatible languages such as C++ and Objective C; the runtime facilities of other programming languages use the C library to access the underlying operating system. In GNU/Linux systems, the C library works with the Linux kernel to implement the operating system behavior seen by user applications. In GNU/Hurd systems, it works with a microkernel and Hurd servers. The GNU C Library implements much of the POSIX.1 functionality in the GNU/Hurd system, using configurations i[4567]86-*-gnu. The current GNU/Hurd support requires out-of-tree patches that will eventually be incorporated into an official GNU C Library release. When working with Linux kernels, this version of the GNU C Library requires Linux kernel version 3.2 or later on all architectures except i[4567]86 and x86_64, where Linux kernel version 2.6.32 or later suffices. Also note that the shared version of the libgcc_s library must be installed for the pthread library to work correctly. The GNU C Library supports these configurations for using Linux kernels: aarch64*-*-linux-gnu alpha*-*-linux-gnu arm-*-linux-gnueabi hppa-*-linux-gnu Not currently functional without patches. i[4567]86-*-linux-gnu x86_64-*-linux-gnu Can build either x86_64 or x32 ia64-*-linux-gnu m68k-*-linux-gnu microblaze*-*-linux-gnu mips-*-linux-gnu mips64-*-linux-gnu powerpc-*-linux-gnu Hardware or software floating point, BE only. powerpc64*-*-linux-gnu Big-endian and little-endian. s390-*-linux-gnu s390x-*-linux-gnu sh[34]-*-linux-gnu sparc*-*-linux-gnu sparc64*-*-linux-gnu tilegx-*-linux-gnu tilepro-*-linux-gnu If you are interested in doing a port, please contact the glibc maintainers; see http://www.gnu.org/software/libc/ for more information. See the file INSTALL to find out how to configure, build, and install the GNU C Library. You might also consider reading the WWW pages for the C library at http://www.gnu.org/software/libc/. The GNU C Library is (almost) completely documented by the Texinfo manual found in the `manual/' subdirectory. The manual is still being updated and contains some known errors and omissions; we regret that we do not have the resources to work on the manual as much as we would like. For corrections to the manual, please file a bug in the `manual' component, following the bug-reporting instructions below. Please be sure to check the manual in the current development sources to see if your problem has already been corrected. Please see http://www.gnu.org/software/libc/bugs.html for bug reporting information. We are now using the Bugzilla system to track all bug reports. This web page gives detailed information on how to report bugs properly. The GNU C Library is free software. See the file COPYING.LIB for copying conditions, and LICENSES for notices about a few contributions that require these additional notices to be distributed. License copyright years may be listed using range notation, e.g., 1996-2015, indicating that every year in the range, inclusive, is a copyrightable year that would otherwise be listed individually.