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Use std::atomic for RefCounter

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Bug: webrtc:9305
Change-Id: I289221357804ed1db9cb07f425cb0f6607b9fe97
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/160409
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29917}
This commit is contained in:
Danil Chapovalov 2019-11-26 12:27:00 +01:00 committed by Commit Bot
parent ef4ab7682b
commit fdaba6cf16
2 changed files with 26 additions and 6 deletions
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1
rtc_base/BUILD.gn
View file

@ -176,7 +176,6 @@ rtc_source_set("refcount") {
"ref_counter.h",
]
deps = [
":atomicops",
":macromagic",
]
}

31
rtc_base/ref_counter.h
View file

@ -10,7 +10,8 @@
#ifndef RTC_BASE_REF_COUNTER_H_
#define RTC_BASE_REF_COUNTER_H_
#include "rtc_base/atomic_ops.h"
#include <atomic>
#include "rtc_base/ref_count.h"
namespace webrtc {
@ -21,7 +22,12 @@ class RefCounter {
explicit RefCounter(int ref_count) : ref_count_(ref_count) {}
RefCounter() = delete;
void IncRef() { rtc::AtomicOps::Increment(&ref_count_); }
void IncRef() {
// Relaxed memory order: The current thread is allowed to act on the
// resource protected by the reference counter both before and after the
// atomic op, so this function doesn't prevent memory access reordering.
ref_count_.fetch_add(1, std::memory_order_relaxed);
}
// Returns kDroppedLastRef if this call dropped the last reference; the caller
// should therefore free the resource protected by the reference counter.
@ -29,7 +35,18 @@ class RefCounter {
// some other caller may have dropped the last reference by the time this call
// returns; all we know is that we didn't do it).
rtc::RefCountReleaseStatus DecRef() {
return (rtc::AtomicOps::Decrement(&ref_count_) == 0)
// Use release-acquire barrier to ensure all actions on the protected
// resource are finished before the resource can be freed.
// When ref_count_after_subtract > 0, this function require
// std::memory_order_release part of the barrier.
// When ref_count_after_subtract == 0, this function require
// std::memory_order_acquire part of the barrier.
// In addition std::memory_order_release is used for synchronization with
// the HasOneRef function to make sure all actions on the protected resource
// are finished before the resource is assumed to have exclusive access.
int ref_count_after_subtract =
ref_count_.fetch_sub(1, std::memory_order_acq_rel) - 1;
return ref_count_after_subtract == 0
? rtc::RefCountReleaseStatus::kDroppedLastRef
: rtc::RefCountReleaseStatus::kOtherRefsRemained;
}
@ -41,11 +58,15 @@ class RefCounter {
// needed for the owning thread to act on the resource protected by the
// reference counter, knowing that it has exclusive access.
bool HasOneRef() const {
return rtc::AtomicOps::AcquireLoad(&ref_count_) == 1;
// To ensure resource protected by the reference counter has exclusive
// access, all changes to the resource before it was released by other
// threads must be visible by current thread. That is provided by release
// (in DecRef) and acquire (in this function) ordering.
return ref_count_.load(std::memory_order_acquire) == 1;
}
private:
volatile int ref_count_;
std::atomic<int> ref_count_;
};
} // namespace webrtc_impl