RFR: 8268638: semaphores of AsyncLogWriter may be broken when JVM is exiting. [v2]

[David Holmes]

On Tue, 15 Jun 2021 01:55:28 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Thanks.  I remove Semaphore::value() and declare friend class SemaphoreLocker there. 
>> 
>> My intention is to do a sanity check. A semaphore can be used to implement a low-level lock if and only if its value is 1. It means that only one thread can enter the critical region at any time.  However, it's error-prone for the RAII class to accept any arbitrary semaphore. It's because its value could be any non-negative number.
>
> I guess I wasn't clear enough.
> 
> Please do not make any changes to the Semaphore class. This "value" is not needed in general and not needed for your sanity checking.
> 
> The Semaphore _lock is defined in the AsyncLogWriter class and initialized to 1 so it can be used to mimic a mutual exclusion lock. This does not need to be sanity checked as it is all handled within a small chunk of code. The AsyncLogLocker does not take a semaphore, it is a friend of AsyncLogWriter and accesses _lock directly.
> 
> Thanks,
> David

PS. If it were warranted, the way to do your "sanity check" would be to introduce a BinarySemaphore class that always initializes to a count of 1. But we do not need this.

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PR: https://git.openjdk.java.net/jdk/pull/4479