RFR (S): 8228857: Refactor PlatformMonitor into PlatformMutex and PlatformMonitor

[David Holmes]

Hi Per,

On 6/08/2019 5:42 pm, Per Liden wrote:
> Hi David,
> 
> On 8/2/19 1:03 AM, David Holmes wrote:
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8228857
>> webrev: http://cr.openjdk.java.net/~dholmes/8228857/webrev/
> 
> src/hotspot/os/posix/os_posix.hpp
> ---------------------------------
> As far as I recall, the macos bug is only related to the lifecycle of 
> condvars (not mutexes). Is that correct?

Not quite. The issue is the allocation of a pthread_mutex_t at an 
address that previously held a pthread_condvar_t.

> If so, PlatformMutex could just always own a plain pthread_mutex_t, and 
> the PLATFORM_MONITOR_IMPL_INDIRECT stuff could be moved to 
> PlatformMonitor to and only deal with the lifecycle of the 
> pthread_cond_t. That way the PlatformMutex would not carry the 
> burden/cost for working around the condvar bug, which would make 
> PlatformMutex even more attractive as a replacement/backing for ZLocks.
> How does that sound?

I deliberately took the simplest path to just leave the macOS workaround 
as is, knowing it means we still pay for the condvar slot on that 
platform. :)

Because the bug involves both types I don't think I can constrain the 
workaround to only the PlatformMonitor class, as I'd need to control the 
allocation in the PlatformMutex as well. I could do that with defining 
and instantiating a different Impl type in PlatformMonitor to be 
installed by PlatformMutex, but the overall complexity increase doesn't 
seem like a good trade off to me. Perhaps I've missed a simple way to 
deal with this?

Thanks,
David

> cheers,
> Per
> 
>>
>> As suggested by Per Liden when PlatformMonitor was introduced by 
>> JDK-8210832 we should refactor PlatformMonitor into a simpler 
>> PlatformMutex extended by PlatformMonitor. That would potentially 
>> allow other synchronization objects e.g. Zlocks, to be replaced by a 
>> suitable PlatformX class, and allows us to redefine JVM_RawMonitor 
>> support to use it (forthcoming change).
>>
>> The refactoring would be obvious and simple if not for the macOS 
>> PThread allocation bug workaround. I had to change the Posix variant 
>> so that we only use the Impl helper class on macOS, so that we can 
>> separate the allocation of the pthread_mutex_t and pthread_cond_t. For 
>> macOS both get allocated in the PlatformMutex.
>>
>> There are not actual usage changes in this RFE.
>>
>> Testing: mach5 tiers 1-3
>>
>> Thanks,
>> David
>> -----