On Thu, Nov 27, 2025 at 4:35 PM Jonathan Wakely <[email protected]> wrote:
> The __futex_wait_flags scoped enum doesn't really have any benefit in
> this file, because this code is no longer in the <atomic> header and so
> we don't need to worry so much about namespace pollution. Just defining
> the constants as int (and locally in the functions where they're needed)
> avoids needing a static_cast<int> from the enum type.
>
> I also noticed that _GLIBCXX_HAVE_LINUX_FUTEX_PRIVATE was never defined,
> which meant we never used the FUTEX_PRIVATE_FLAG to tell the kernel that
> all futex ops are process-private.
>
> Also add comments and deleted definitions describing the API expected
> for targets that define _GLIBCXX_HAVE_PLATFORM_WAIT.
>
> libstdc++-v3/ChangeLog:
>
> * src/c++20/atomic.cc: Document platform wait API.
> (__futex_wait_flags): Remove enumeration type.
> (futex_private_flag): Define constant for FUTEX_PRIVATE_FLAG.
> (__platform_wait): Use local variables for futex op constants.
> (__platform_notify): Likewise.
> (__platform_wait_until): Likewise. Adjust parameter types for
> consistency with __platform_wait.
> ---
>
> Tested x86_64-linux.
>
LGTM.
>
> libstdc++-v3/src/c++20/atomic.cc | 98 ++++++++++++++++++++------------
> 1 file changed, 63 insertions(+), 35 deletions(-)
>
> diff --git a/libstdc++-v3/src/c++20/atomic.cc
> b/libstdc++-v3/src/c++20/atomic.cc
> index 80915617f0bf..fdd67d834768 100644
> --- a/libstdc++-v3/src/c++20/atomic.cc
> +++ b/libstdc++-v3/src/c++20/atomic.cc
> @@ -35,8 +35,8 @@
> #include <bits/functional_hash.h>
>
> #ifdef _GLIBCXX_HAVE_LINUX_FUTEX
> +# include <sys/syscall.h> // SYS_futex
> # include <unistd.h>
> -# include <syscall.h>
> # include <sys/time.h> // timespec
> # define _GLIBCXX_HAVE_PLATFORM_WAIT 1
> #endif
> @@ -50,56 +50,84 @@ namespace __detail
> {
> namespace
> {
> -#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
> - enum class __futex_wait_flags : int
> - {
> -#ifdef _GLIBCXX_HAVE_LINUX_FUTEX_PRIVATE
> - __private_flag = 128,
> -#else
> - __private_flag = 0,
> -#endif
> - __wait = 0,
> - __wake = 1,
> - __wait_bitset = 9,
> - __wake_bitset = 10,
> - __wait_private = __wait | __private_flag,
> - __wake_private = __wake | __private_flag,
> - __wait_bitset_private = __wait_bitset | __private_flag,
> - __wake_bitset_private = __wake_bitset | __private_flag,
> - __bitset_match_any = -1
> - };
> +#ifndef _GLIBCXX_HAVE_PLATFORM_WAIT
> +
> + // If _GLIBCXX_HAVE_PLATFORM_WAIT is defined then the following three
> + // functions should be defined in terms of platform-specific wait/wake
> + // primitives. The `obj_size` parameter is the size in bytes of the
> object
> + // at `*addr` (used if the platform supports waiting on more than one
> size,
> + // in which case `addr` would be cast to a different type).
> +
> + // Deleted definitions are here to give better errors if these functions
> + // are used when _GLIBCXX_HAVE_PLATFORM_WAIT is not defined.
> +
> + // Wait until *addr != curr_val.
> + // Once a thread is waiting, it will not unblock and notice the value
> + // has changed unless explicitly notified using `__platform_notify`.
> + void
> + __platform_wait(const __platform_wait_t* addr,
> + __platform_wait_t curr_val,
> + int obj_size) = delete;
> +
> + // Wake one thread that is waiting for `*addr` to change,
> + // or all waiting threads if `wake_all` is true.
> + void
> + __platform_notify(const __platform_wait_t* addr,
> + bool wake_all,
> + int obj_size) = delete;
> +
> + // As `__platform_wait` but with timeout.
> + // Returns true if the wait ended before the timeout (which could be
> because
> + // the value changed and __platform_notify was called, but could be
> because
> + // the wait was interrupted by a signal, or just a spurious wake).
> + // Returns false if the timeout was reached.
> + bool
> + __platform_wait_until(const __platform_wait_t* addr,
> + __platform_wait_t curr_val,
> + __wait_clock_t::time_point timeout,
> + int obj_size) = delete;
> +
> +#elif defined _GLIBCXX_HAVE_LINUX_FUTEX
> +
> + const int futex_private_flag = 128;
>
> void
> __platform_wait(const int* addr, int val, int /* obj_size */) noexcept
> {
> - if (syscall(SYS_futex, addr,
> - static_cast<int>(__futex_wait_flags::__wait_private),
> - val, nullptr))
> + const int futex_op_wait = 0;
> + const int futex_op_wait_private = futex_op_wait | futex_private_flag;
> +
> + if (syscall(SYS_futex, addr, futex_op_wait_private, val, nullptr))
> if (errno != EAGAIN && errno != EINTR)
> __throw_system_error(errno);
> }
>
> void
> - __platform_notify(const int* addr, bool all, int) noexcept
> + __platform_notify(const int* addr, bool all, int /* obj_size */)
> noexcept
> {
> - syscall(SYS_futex, addr,
> - static_cast<int>(__futex_wait_flags::__wake_private),
> - all ? INT_MAX : 1);
> + const int futex_op_wake = 1;
> + const int futex_op_wake_private = futex_op_wake | futex_private_flag;
> +
> + syscall(SYS_futex, addr, futex_op_wake_private, all ? INT_MAX : 1);
> }
>
> // returns true if wait ended before timeout
> bool
> - __platform_wait_until(const __platform_wait_t* addr,
> - __platform_wait_t val,
> - const __wait_clock_t::time_point& __atime,
> + __platform_wait_until(const int* addr, int val,
> + const __wait_clock_t::time_point& abs_time,
> int /* obj_size */) noexcept
> {
> - struct timespec timeout = chrono::__to_timeout_timespec(__atime);
> + // FUTEX_WAIT expects a relative timeout, so must use
> FUTEX_WAIT_BITSET
> + // for an absolute timeout.
> + const int futex_op_wait_bitset = 9;
> + const int futex_op_wait_bitset_private
> + = futex_op_wait_bitset | futex_private_flag;
> + const int futex_bitset_match_any = 0xffffffff;
>
> - if (syscall(SYS_futex, addr,
> -
> static_cast<int>(__futex_wait_flags::__wait_bitset_private),
> - val, &timeout, nullptr,
> - static_cast<int>(__futex_wait_flags::__bitset_match_any)))
> + struct timespec timeout = chrono::__to_timeout_timespec(abs_time);
> +
> + if (syscall(SYS_futex, addr, futex_op_wait_bitset_private, val,
> + &timeout, nullptr, futex_bitset_match_any))
> {
> if (errno == ETIMEDOUT)
> return false;
> @@ -108,7 +136,7 @@ namespace
> }
> return true;
> }
> -#endif // HAVE_LINUX_FUTEX
> +#endif // HAVE_PLATFORM_WAIT
>
> // The state used by atomic waiting and notifying functions.
> struct __waitable_state
> --
> 2.51.1
>
>
