libc/src/__support/threads/linux/futex_utils.h

   1 //===--- Futex Wrapper ------------------------------------------*- C++ -*-===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_FUTEX_UTILS_H
  10 #define LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_FUTEX_UTILS_H
  11
  12 #include "src/__support/CPP/atomic.h"
  13 #include "src/__support/CPP/limits.h"
  14 #include "src/__support/CPP/optional.h"
  15 #include "src/__support/OSUtil/syscall.h"
  16 #include "src/__support/macros/attributes.h"
  17 #include "src/__support/macros/config.h"
  18 #include "src/__support/threads/linux/futex_word.h"
  19 #include "src/__support/time/linux/abs_timeout.h"
  20 #include <linux/errno.h>
  21 #include <linux/futex.h>
  22
  23 namespace LIBC_NAMESPACE_DECL {
  24 class Futex : public cpp::Atomic<FutexWordType> {
  25 public:
  26   using Timeout = internal::AbsTimeout;
  27   LIBC_INLINE constexpr Futex(FutexWordType value)
  28       : cpp::Atomic<FutexWordType>(value) {}
  29   LIBC_INLINE Futex &operator=(FutexWordType value) {
  30     cpp::Atomic<FutexWordType>::store(value);
  31     return *this;
  32   }
  33   LIBC_INLINE long wait(FutexWordType expected,
  34                         cpp::optional<Timeout> timeout = cpp::nullopt,
  35                         bool is_shared = false) {
  36     // use bitset variants to enforce abs_time
  37     uint32_t op = is_shared ? FUTEX_WAIT_BITSET : FUTEX_WAIT_BITSET_PRIVATE;
  38     if (timeout && timeout->is_realtime()) {
  39       op |= FUTEX_CLOCK_REALTIME;
  40     }
  41     for (;;) {
  42       if (this->load(cpp::MemoryOrder::RELAXED) != expected)
  43         return 0;
  44
  45       long ret = syscall_impl<long>(
  46           /* syscall number */ FUTEX_SYSCALL_ID,
  47           /* futex address */ this,
  48           /* futex operation  */ op,
  49           /* expected value */ expected,
  50           /* timeout */ timeout ? &timeout->get_timespec() : nullptr,
  51           /* ignored */ nullptr,
  52           /* bitset */ FUTEX_BITSET_MATCH_ANY);
  53
  54       // continue waiting if interrupted; otherwise return the result
  55       // which should normally be 0 or -ETIMEOUT
  56       if (ret == -EINTR)
  57         continue;
  58
  59       return ret;
  60     }
  61   }
  62   LIBC_INLINE long notify_one(bool is_shared = false) {
  63     return syscall_impl<long>(
  64         /* syscall number */ FUTEX_SYSCALL_ID,
  65         /* futex address */ this,
  66         /* futex operation  */ is_shared ? FUTEX_WAKE : FUTEX_WAKE_PRIVATE,
  67         /* wake up limit */ 1,
  68         /* ignored */ nullptr,
  69         /* ignored */ nullptr,
  70         /* ignored */ 0);
  71   }
  72   LIBC_INLINE long notify_all(bool is_shared = false) {
  73     return syscall_impl<long>(
  74         /* syscall number */ FUTEX_SYSCALL_ID,
  75         /* futex address */ this,
  76         /* futex operation  */ is_shared ? FUTEX_WAKE : FUTEX_WAKE_PRIVATE,
  77         /* wake up limit */ cpp::numeric_limits<int>::max(),
  78         /* ignored */ nullptr,
  79         /* ignored */ nullptr,
  80         /* ignored */ 0);
  81   }
  82 };
  83
  84 static_assert(__is_standard_layout(Futex),
  85               "Futex must be a standard layout type.");
  86 } // namespace LIBC_NAMESPACE_DECL
  87
  88 #endif // LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_FUTEX_UTILS_H