[RFC PATCH 0/2] Add a futex interface to glibc (sys/futex.h)

[Adhemerval Zanella]

This patchset adds wrappers for the futex system call through a new
<sys/futex.h> header.  They are futex_wait, futex_timedwait,
futex_wake, futex_requeue, and futex_waitv.

Exporting a futex interface from libc is a long-standing request, as
the futex syscall is the blocking building block for essentially
every custom synchronization primitive on Linux.  And since glibc
provides no wrapper, every project ends up hand-rolling it; with
per-architecture syscall assembly, ad-hoc time64 syscall selection,
and workarounds for the syscall's historical quirks duplicated in
each of them (libstdc++, libc++, libgomp, libitm, the Go runtime,
OpenMP, allocators, and so on).

It has also become relevant as the kernel expands the futex interface,
where futex_waitv and the futex2 syscall family only make sense to
expose from libc if there is a consistent futex API to build on.

This patchset provides that base operations plus futex_waitv, sharing
one set of conventions (flags following the kernel FUTEX_PRIVATE_FLAG,
absolute timeouts with an explicit clockid, -1/errno error reporting,
no cancellation points), so future futex2 additions are incremental
rather than a new design.

Only the well-defined subset of the kernel interface is exposed (the
racy unchecked FUTEX_REQUEUE, the removed FUTEX_FD, FUTEX_WAKE_OP,
bitset matching, and the PI operations are left out).  The
futex_timedwait supports both time_t sizes with the usual TIME_BITS=64
redirection; futex_waitv mirrors the kernel ABI directly (struct layout,
argument order, FUTEX2* per-waiter flags) and is 64-bit time_t only.
The wrappers compile down to the same code as a hand-rolled syscall
(the waits and requeue are a flags check plus a tail call; wake
inlines the syscall), so there is no cost to adopting them.

To verify the interface's usability, I built a proof of concept that
converts the two futex users in GCC (libitm and libgomp) to the new
functions.  The required work is minimal: libitm's wrappers become
three-line calls, and libgomp only needs an asm-label binding because
its internal wrappers reuse the  utex_wait/futex_wake names.

In both cases the per-architecture syscall assembly and the obsolete
private-to-shared runtime fallbacks become unnecessary.  A survey of
the remaining futex users in GCC and LLVM (libstdc++, libc++, libgo,
OpenMP, scudo) showed that all of their usage should map onto this
interface as well.

Adhemerval Zanella (2):
  linux: Add futex_wait, futex_timedwait, futex_wake, and futex_requeue
  linux: Add futex_waitv

 NEWS                                          |   9 +-
 include/sys/futex.h                           |  32 +++
 manual/threads.texi                           | 211 ++++++++++++++++++
 nptl/futex-internal.c                         |  32 +++
 sysdeps/nptl/futex-internal.h                 |  58 +++--
 sysdeps/nptl/lowlevellock-futex.h             |  12 +
 sysdeps/unix/sysv/linux/Makefile              |  20 ++
 sysdeps/unix/sysv/linux/Versions              |  10 +
 sysdeps/unix/sysv/linux/aarch64/libc.abilist  |   5 +
 sysdeps/unix/sysv/linux/alpha/libc.abilist    |   5 +
 sysdeps/unix/sysv/linux/arc/libc.abilist      |   5 +
 sysdeps/unix/sysv/linux/arm/be/libc.abilist   |   6 +
 sysdeps/unix/sysv/linux/arm/le/libc.abilist   |   6 +
 sysdeps/unix/sysv/linux/csky/libc.abilist     |   6 +
 sysdeps/unix/sysv/linux/futex_requeue.c       |  35 +++
 sysdeps/unix/sysv/linux/futex_timedwait.c     |  55 +++++
 sysdeps/unix/sysv/linux/futex_wait.c          |  36 +++
 sysdeps/unix/sysv/linux/futex_waitv.c         |  38 ++++
 sysdeps/unix/sysv/linux/futex_wake.c          |  38 ++++
 sysdeps/unix/sysv/linux/hppa/libc.abilist     |   6 +
 sysdeps/unix/sysv/linux/i386/libc.abilist     |   6 +
 .../sysv/linux/loongarch/ilp32/libc.abilist   |   5 +
 .../sysv/linux/loongarch/lp64/libc.abilist    |   5 +
 .../sysv/linux/m68k/coldfire/libc.abilist     |   6 +
 .../unix/sysv/linux/m68k/m680x0/libc.abilist  |   6 +
 .../sysv/linux/microblaze/be/libc.abilist     |   6 +
 .../sysv/linux/microblaze/le/libc.abilist     |   6 +
 .../sysv/linux/mips/mips32/fpu/libc.abilist   |   6 +
 .../sysv/linux/mips/mips32/nofpu/libc.abilist |   6 +
 .../sysv/linux/mips/mips64/n32/libc.abilist   |   6 +
 .../sysv/linux/mips/mips64/n64/libc.abilist   |   5 +
 sysdeps/unix/sysv/linux/or1k/libc.abilist     |   5 +
 .../linux/powerpc/powerpc32/fpu/libc.abilist  |   6 +
 .../powerpc/powerpc32/nofpu/libc.abilist      |   6 +
 .../linux/powerpc/powerpc64/be/libc.abilist   |   5 +
 .../linux/powerpc/powerpc64/le/libc.abilist   |   5 +
 .../unix/sysv/linux/riscv/rv32/libc.abilist   |   5 +
 .../unix/sysv/linux/riscv/rv64/libc.abilist   |   5 +
 sysdeps/unix/sysv/linux/s390/libc.abilist     |   5 +
 sysdeps/unix/sysv/linux/sh/be/libc.abilist    |   6 +
 sysdeps/unix/sysv/linux/sh/le/libc.abilist    |   6 +
 .../sysv/linux/sparc/sparc32/libc.abilist     |   6 +
 .../sysv/linux/sparc/sparc64/libc.abilist     |   5 +
 sysdeps/unix/sysv/linux/sys/futex.h           | 135 +++++++++++
 sysdeps/unix/sysv/linux/tst-futex-consts.py   |  71 ++++++
 sysdeps/unix/sysv/linux/tst-futex-time64.c    |   1 +
 .../unix/sysv/linux/tst-futex-waitv-time64.c  |   1 +
 sysdeps/unix/sysv/linux/tst-futex-waitv.c     | 197 ++++++++++++++++
 sysdeps/unix/sysv/linux/tst-futex.c           | 179 +++++++++++++++
 .../unix/sysv/linux/x86_64/64/libc.abilist    |   5 +
 .../unix/sysv/linux/x86_64/x32/libc.abilist   |   5 +
 51 files changed, 1323 insertions(+), 24 deletions(-)
 create mode 100644 include/sys/futex.h
 create mode 100644 sysdeps/unix/sysv/linux/futex_requeue.c
 create mode 100644 sysdeps/unix/sysv/linux/futex_timedwait.c
 create mode 100644 sysdeps/unix/sysv/linux/futex_wait.c
 create mode 100644 sysdeps/unix/sysv/linux/futex_waitv.c
 create mode 100644 sysdeps/unix/sysv/linux/futex_wake.c
 create mode 100644 sysdeps/unix/sysv/linux/sys/futex.h
 create mode 100644 sysdeps/unix/sysv/linux/tst-futex-consts.py
 create mode 100644 sysdeps/unix/sysv/linux/tst-futex-time64.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-futex-waitv-time64.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-futex-waitv.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-futex.c

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2.53.0