text: // Copyright 2017 The Abseil Authors.
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //      https://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  //
  // Core interfaces and definitions used by by low-level interfaces such as
  // SpinLock.
  
  #ifndef ABSL_BASE_INTERNAL_LOW_LEVEL_SCHEDULING_H_
  #define ABSL_BASE_INTERNAL_LOW_LEVEL_SCHEDULING_H_
  
  #include "absl/base/internal/scheduling_mode.h"
  #include "absl/base/macros.h"
  
  // The following two declarations exist so SchedulingGuard may friend them with
  // the appropriate language linkage.  These callbacks allow libc internals, such
  // as function level statics, to schedule cooperatively when locking.
  extern "C" bool __google_disable_rescheduling(void);
  extern "C" void __google_enable_rescheduling(bool disable_result);
  
  namespace absl {
  inline namespace lts_2019_08_08 {
  namespace base_internal {
  
  class SchedulingHelper;  // To allow use of SchedulingGuard.
  class SpinLock;          // To allow use of SchedulingGuard.
  
  // SchedulingGuard
  // Provides guard semantics that may be used to disable cooperative rescheduling
  // of the calling thread within specific program blocks.  This is used to
  // protect resources (e.g. low-level SpinLocks or Domain code) that cooperative
  // scheduling depends on.
  //
  // Domain implementations capable of rescheduling in reaction to involuntary
  // kernel thread actions (e.g blocking due to a pagefault or syscall) must
  // guarantee that an annotated thread is not allowed to (cooperatively)
  // reschedule until the annotated region is complete.
  //
  // It is an error to attempt to use a cooperatively scheduled resource (e.g.
  // Mutex) within a rescheduling-disabled region.
  //
  // All methods are async-signal safe.
  class SchedulingGuard {
   public:
    // Returns true iff the calling thread may be cooperatively rescheduled.
    static bool ReschedulingIsAllowed();
  
   private:
    // Disable cooperative rescheduling of the calling thread.  It may still
    // initiate scheduling operations (e.g. wake-ups), however, it may not itself
    // reschedule.  Nestable.  The returned result is opaque, clients should not
    // attempt to interpret it.
    // REQUIRES: Result must be passed to a pairing EnableScheduling().
    static bool DisableRescheduling();
  
    // Marks the end of a rescheduling disabled region, previously started by
    // DisableRescheduling().
    // REQUIRES: Pairs with innermost call (and result) of DisableRescheduling().
    static void EnableRescheduling(bool disable_result);
  
    // A scoped helper for {Disable, Enable}Rescheduling().
    // REQUIRES: destructor must run in same thread as constructor.
    struct ScopedDisable {
      ScopedDisable() { disabled = SchedulingGuard::DisableRescheduling(); }
      ~ScopedDisable() { SchedulingGuard::EnableRescheduling(disabled); }
  
      bool disabled;
    };
  
    // Access to SchedulingGuard is explicitly white-listed.
    friend class SchedulingHelper;
    friend class SpinLock;
  
    SchedulingGuard(const SchedulingGuard&) = delete;
    SchedulingGuard& operator=(const SchedulingGuard&) = delete;
  };
  
  //------------------------------------------------------------------------------
  // End of public interfaces.
  //------------------------------------------------------------------------------
  
  inline bool SchedulingGuard::ReschedulingIsAllowed() {
    return false;
  }
  
  inline bool SchedulingGuard::DisableRescheduling() {
    return false;
  }
  
  inline void SchedulingGuard::EnableRescheduling(bool /* disable_result */) {
    return;
  }
  
  }  // namespace base_internal
  }  // inline namespace lts_2019_08_08
  }  // namespace absl
  
  #endif  // ABSL_BASE_INTERNAL_LOW_LEVEL_SCHEDULING_H_
  