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// 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// 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.
#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 {
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 {
// Returns true iff the calling thread may be cooperatively rescheduled.
static bool ReschedulingIsAllowed();
// 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 */) {
} // namespace base_internal
} // namespace absl