Bug 1429647 Switch privacy.reduceTimerPrecision and privacy.resistFingerprinting to use Relaxed Semantics r=froydnj,mystor

MozReview-Commit-ID: HdW4tmQAcTT

--HG--
extra : rebase_source : c1e1c44a79cb0449da22c8fce954da078c1972e4

js/src/jsdate.cpp

@@ -54,7 +54,7 @@ using mozilla::ArrayLength;
 using mozilla::IsFinite;
 using mozilla::IsNaN;
 using mozilla::NumbersAreIdentical;
-using mozilla::ReleaseAcquire;
+using mozilla::Relaxed;
 
 using JS::AutoCheckCannotGC;
 using JS::ClippedTime;
@@ -63,7 +63,7 @@ using JS::TimeClip;
 using JS::ToInteger;
 
 // When this value is non-zero, we'll round the time by this resolution.
-static Atomic<uint32_t, ReleaseAcquire> sResolutionUsec;
+static Atomic<uint32_t, Relaxed> sResolutionUsec;
 
 /*
  * The JS 'Date' object is patterned after the Java 'Date' object.

toolkit/components/resistfingerprinting/nsRFPService.cpp

@@ -58,12 +58,22 @@ static mozilla::LazyLogModule gResistFingerprintingLog("nsResistFingerprinting")
 
 NS_IMPL_ISUPPORTS(nsRFPService, nsIObserver)
 
+/*
+ * The below variables are marked with 'Relaxed' memory ordering. We don't
+ * particurally care that threads have a percently consistent view of the values
+ * of these prefs. They are not expected to change often, and having an outdated
+ * view is not particurally harmful. They will eventually become consistent.
+ *
+ * The variables will, however, be read often (specifically sResolutionUSec on
+ * each timer rounding) so performance is important.
+ */
+
 static StaticRefPtr<nsRFPService> sRFPService;
 static bool sInitialized = false;
-Atomic<bool, ReleaseAcquire> nsRFPService::sPrivacyResistFingerprinting;
-Atomic<bool, ReleaseAcquire> nsRFPService::sPrivacyTimerPrecisionReduction;
+Atomic<bool, Relaxed> nsRFPService::sPrivacyResistFingerprinting;
+Atomic<bool, Relaxed> nsRFPService::sPrivacyTimerPrecisionReduction;
 // Note: anytime you want to use this variable, you should probably use TimerResolution() instead
-Atomic<uint32_t, ReleaseAcquire> sResolutionUSec;
+Atomic<uint32_t, Relaxed> sResolutionUSec;
 static uint32_t sVideoFramesPerSec;
 static uint32_t sVideoDroppedRatio;
 static uint32_t sTargetVideoRes;

toolkit/components/resistfingerprinting/nsRFPService.h

@@ -247,8 +247,8 @@ private:
                                     const WidgetKeyboardEvent* aKeyboardEvent,
                                     SpoofingKeyboardCode& aOut);
 
-  static Atomic<bool, ReleaseAcquire> sPrivacyResistFingerprinting;
-  static Atomic<bool, ReleaseAcquire> sPrivacyTimerPrecisionReduction;
+  static Atomic<bool, Relaxed> sPrivacyResistFingerprinting;
+  static Atomic<bool, Relaxed> sPrivacyTimerPrecisionReduction;
 
   static nsDataHashtable<KeyboardHashKey, const SpoofingKeyboardCode*>* sSpoofingKeyboardCodes;