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GNU C Library master sources
1db84775f8
This fixes the lost wakeup (from a bug in signal stealing) with a change in the usage of g_signals[] in the condition variable internal state. It also completely eliminates the concept and handling of signal stealing, as well as the need for signalers to block to wait for waiters to wake up every time there is a G1/G2 switch. This greatly reduces the average and maximum latency for pthread_cond_signal. The g_signals[] field now contains a signal count that is relative to the current g1_start value. Since it is a 32-bit field, and the LSB is still reserved (though not currently used anymore), it has a 31-bit value that corresponds to the low 31 bits of the sequence number in g1_start. (since g1_start also has an LSB flag, this means bits 31:1 in g_signals correspond to bits 31:1 in g1_start, plus the current signal count) By making the signal count relative to g1_start, there is no longer any ambiguity or A/B/A issue, and thus any checks before blocking, including the futex call itself, are guaranteed not to block if the G1/G2 switch occurs, even if the signal count remains the same. This allows initially safely blocking in G2 until the switch to G1 occurs, and then transitioning from G1 to a new G1 or G2, and always being able to distinguish the state change. This removes the race condition and A/B/A problems that otherwise ocurred if a late (pre-empted) waiter were to resume just as the futex call attempted to block on g_signal since otherwise there was no last opportunity to re-check things like whether the current G1 group was already closed. By fixing these issues, the signal stealing code can be eliminated, since there is no concept of signal stealing anymore. The code to block for all waiters to exit g_refs can also be removed, since any waiters that are still in the g_refs region can be guaranteed to safely wake up and exit. If there are still any left at this time, they are all sent one final futex wakeup to ensure that they are not blocked any longer, but there is no need for the signaller to block and wait for them to wake up and exit the g_refs region. The signal count is then effectively "zeroed" but since it is now relative to g1_start, this is done by advancing it to a new value that can be observed by any pending blocking waiters. Any late waiters can always tell the difference, and can thus just cleanly exit if they are in a stale G1 or G2. They can never steal a signal from the current G1 if they are not in the current G1, since the signal value that has to match in the cmpxchg has the low 31 bits of the g1_start value contained in it, and that's first checked, and then it won't match if there's a G1/G2 change. Note: the 31-bit sequence number used in g_signals is designed to handle wrap-around when checking the signal count, but if the entire 31-bit wraparound (2 billion signals) occurs while there is still a late waiter that has not yet resumed, and it happens to then match the current g1_start low bits, and the pre-emption occurs after the normal "closed group" checks (which are 64-bit) but then hits the futex syscall and signal consuming code, then an A/B/A issue could still result and cause an incorrect assumption about whether it should block. This particular scenario seems unlikely in practice. Note that once awake from the futex, the waiter would notice the closed group before consuming the signal (since that's still a 64-bit check that would not be aliased in the wrap-around in g_signals), so the biggest impact would be blocking on the futex until the next full wakeup from a G1/G2 switch. Signed-off-by: Frank Barrus <frankbarrus_sw@shaggy.cc> Reviewed-by: Carlos O'Donell <carlos@redhat.com> |
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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 and x86_64-gnu. When working with Linux kernels, this version of the GNU C Library requires Linux kernel version 3.2 or later. 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 arc*-*-linux-gnu arm-*-linux-gnueabi csky-*-linux-gnuabiv2 hppa-*-linux-gnu i[4567]86-*-linux-gnu x86_64-*-linux-gnu Can build either x86_64 or x32 loongarch64-*-linux-gnu Hardware floating point, LE only. m68k-*-linux-gnu microblaze*-*-linux-gnu mips-*-linux-gnu mips64-*-linux-gnu or1k-*-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 riscv32-*-linux-gnu riscv64-*-linux-gnu sh[34]-*-linux-gnu sparc*-*-linux-gnu sparc64*-*-linux-gnu If you are interested in doing a port, please contact the glibc maintainers; see https://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 https://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 https://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.