RFR: 8221360: Eliminate Shared_DirtyCardQ_lock [v3]

[Thomas Schatzl]

On Tue, 24 Aug 2021 13:09:52 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Please review this change to the handling of a very rare race condition in
>> concurrent refinement.  There is a situation where we many be unable to
>> process a card and need to re-dirty and re-enqueue it for later processing.
>> 
>> The old code handled this using a shared dirty card queue, which various
>> other places needed to know about as well.  This was the user of the
>> Shared_DirtyCardQ_lock.  It was the only remaining Mutex whose rank is based
>> on Mutex::access.
>> 
>> An option for eliminating that lock was to use a lock-free queue.  But
>> rather than take on that complexity, we instead brute-force the problem.
>> The new implementation simply allocates and enqueues an entire buffer for
>> the one card that needs to be reprocessed.  Those operations are already
>> lock-free.  The extra cost doesn't really matter, since this situation
>> almost never arises.
>> 
>> This is now done inline in refine_card_concurrently.  It's not worth the
>> overhead of a class for what is now just a few lines of code needed in
>> exactly one place.  Places that knew about the shared queue in order to
>> flush/reset it at certain points no longer have such responsibilities.
>> 
>> This change also renders JDK-8214997 moot, since there aren't any 'access'
>> locks.
>> https://bugs.openjdk.java.net/browse/JDK-8214997
>> Crash holding 'access' lock can deadlock in JVMCI compiler thread
>> 
>> This change also allows the removal of Mutex::access.  That's being left to
>> a new enhancement, since I think it also allows removing Mutex::tty,
>> reverting back to using Mutex::event for the tty_lock.  That is, essentially
>> undo JDK-8214315.
>> https://bugs.openjdk.java.net/browse/JDK-8272480
>> 
>> Testing:
>> mach5 tier1-5
>
> Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:
> 
>   helper function for re-enqueue

Still good, thanks.

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Marked as reviewed by tschatzl (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/5118