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authorGravatar misterg <misterg@google.com>2017-09-19 16:54:40 -0400
committerGravatar misterg <misterg@google.com>2017-09-19 16:54:40 -0400
commitc2e754829628d1e9b7a16b3389cfdace76950fdf (patch)
tree5a7f056f44e27c30e10025113b644f0b3b5801fc /absl/base/internal/low_level_scheduling.h
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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
+//
+// http://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 //base 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 {
+namespace base_internal {
+
+class SpinLock; // To allow use of SchedulingGuard.
+class SchedulingHelper; // 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
+} // namespace absl
+#endif // ABSL_BASE_INTERNAL_LOW_LEVEL_SCHEDULING_H_