RFR: 8145083: Use semaphore instead of mutex for synchronization of Unified Logging configuration

[Markus Gronlund]

Hi Marcus,

Thanks for updating the patch.

I think you could also move the lock representation, i.e the Semaphore to become a (static) field inside the ConfigurationLocker class - this since you want all entry/exits to go through instances of this object.

I know that the other MutexLocker's don't aggregate their lock resource but rather take them as pointer in their constructors. However, I don’t think emulating that behavior is paramount here, so if the semaphore moves into the ConfigurationLocker, another name (for discernment against the other *Lockers) could possibly be used - for example: "ConfigurationLock" which is generic enough for the abstraction letting you change how locking is done underneath should you need to.

Thanks
Markus




-----Original Message-----
From: Marcus Larsson 
Sent: den 14 december 2015 16:31
To: serviceability-dev at openjdk.java.net
Cc: hotspot-runtime-dev at openjdk.java.net
Subject: Re: RFR: 8145083: Use semaphore instead of mutex for synchronization of Unified Logging configuration

Hi again,

Made some changes to the patch after feedback from Markus Grönlund. The ConfigurationLocker and the semaphore have been moved out of the LogConfiguration.hpp and into the .cpp instead. The lock is only used internally by the LogConfiguration so this makes sense. It simplifies the LogConfiguration interface and hides the details of the locking under the hood.

New webrev:
http://cr.openjdk.java.net/~mlarsson/8145083/webrev.02/

Incremental:
http://cr.openjdk.java.net/~mlarsson/8145083/webrev.01-02/

Thanks,
Marcus

On 2015-12-14 15:53, Marcus Larsson wrote:
> Hi,
>
> New webrev:
> http://cr.openjdk.java.net/~mlarsson/8145083/webrev.01/
>
> Incremental:
> http://cr.openjdk.java.net/~mlarsson/8145083/webrev.00-01/
>
> Changes:
> * Introduced the ConfigurationLocker class for automatic wait/signal 
> in constructor/destructor just like a MutexLocker.
> * Added an assert to verify that the "lock" is held by the current 
> thread when calling configure_output.
> * Made the config-string functions in LogOutput protected and 
> LogConfiguration a friend of LogOutput to prevent incorrect usage of 
> these functions. These functions should typically only be used inside 
> configure_output, which now ensures that the lock is held.
>
> Thanks,
> Marcus
>
>
> On 2015-12-14 11:13, Marcus Larsson wrote:
>> Hi,
>>
>> Please review the following patch to use a semaphore instead of a 
>> mutex for the synchronization of log configuration. Using a mutex 
>> requires some parts of the VM to be initialized, whereas the 
>> semaphores can be used right from the start. This simplifies the code 
>> and allows very early log configuration without special cases for 
>> early configuration vs reconfiguration after VM init.
>>
>> Webrev:
>> http://cr.openjdk.java.net/~mlarsson/8145083/webrev.00/
>>
>> Issue:
>> https://bugs.openjdk.java.net/browse/JDK-8145083
>>
>> Thanks,
>> Marcus
>