[Question] Can mutex() be simplified this way?

[qiutongs]

guava/guava/src/com/google/common/util/concurrent/RateLimiter.java

Lines 189 to 200 in 8385600

	private Object mutex() {
	Object mutex = mutexDoNotUseDirectly;
	if (mutex == null) {
	synchronized (this) {
	mutex = mutexDoNotUseDirectly;
	if (mutex == null) {
	mutexDoNotUseDirectly = mutex = new Object();
	}
	}
	}
	return mutex;
	}

Basically, my confusion is why the local variable "mutex" is needed.

  private Object mutex() {
    if (mutexDoNotUseDirectly == null) {
      synchronized (this) {
        if (mutexDoNotUseDirectly == null) {
          mutexDoNotUseDirectly = new Object();
        }
      }
    }
    return mutexDoNotUseDirectly;
  }

[JakeWharton]

It avoids an additional volatile read of the field once it's determined to be non-null.

…

On Fri, Feb 8, 2019, 12:48 PM qiutongs ***@***.*** wrote: https://github.com/google/guava/blob/838560034dfaa1afdf51a126afe6b8b8e6cce3dd/guava/src/com/google/common/util/concurrent/RateLimiter.java#L189-L200 Basically, my confusion is why the local variable "mutex" is needed. private Object mutex() { if (mutexDoNotUseDirectly == null) { synchronized (this) { if (mutexDoNotUseDirectly == null) { mutexDoNotUseDirectly = new Object(); } } } return mutexDoNotUseDirectly; } — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#3381>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAEEEVOGLttPeLzBA5hAxCUvvnxpB6dPks5vLbiGgaJpZM4awy17> .

[qiutongs]

@JakeWharton
Thank you for quick reply! Let me make sure I understand you correctly. The additional volatile read you meant is the one in the "return" statement in my code?

So the original code reads "mutexDoNotUseDirectly" on lines 190, 193, 195 (3 times in worst case). But my code reads "mutexDoNotUseDirectly" four times in worst case.

Is that the thing we try to optimize with the local variable?

[JakeWharton]

Yes. Also the common case becomes 1 not 2.

…

On Fri, Feb 8, 2019, 1:05 PM qiutongs ***@***.*** wrote: @JakeWharton <https://github.com/JakeWharton> Thank you for quick reply! Let me make sure I understand you correctly. The additional volatile read you meant is the one in the "return" statement in my code? So the original code reads "mutexDoNotUseDirectly" on lines 190, 193, 195 (3 times in worst case). But my code reads "mutexDoNotUseDirectly" four times in worst case. Is that the thing we try to optimize with the local variable? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#3381 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAEEEbcKI9WXIDIZ6p5sBaC8pBD06m_tks5vLbyFgaJpZM4awy17> .

[qiutongs]

It makes sense. Thank you!