RFR(s): 8204166: TLH: Semaphore may not be destroy until signal have returned.

[Robbin Ehn]

On 06/18/2018 03:07 PM, Robbin Ehn wrote:
> Hi all,
> 
> After some internal discussions I changed the patch to:
> http://rehn-ws.se.oracle.com/cr_mirror/8204166/v2/

Correct external url:
http://cr.openjdk.java.net/~rehn/8204166/v2/

/Robbin

> 
> Which handles thread off javathreads list better.
> 
> Passes handshake testing and ZGC testing seems okay.
> 
> Thanks, Robbin
> 
> On 06/14/2018 12:11 PM, Robbin Ehn wrote:
>> Hi all, please review.
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8204166
>> Webrev: http://cr.openjdk.java.net/~rehn/8204166/v1/webrev/
>>
>> The root cause of this failure is a bug in the posix semaphores: 
>> https://sourceware.org/bugzilla/show_bug.cgi?id=12674
>>
>> Thread a:
>> sem_post(my_sem);
>>
>> Thread b:
>> sem_wait(my_sem);
>> sem_destroy(my_sem);
>>
>> Thread b is waiting on my_sem (count 0), Thread a posts (count 0->1).
>> If Thread b start executing directly after the increment in post but before
>> Thread a leaves the call to post and manage to destroy the semaphore. Thread a
>> _can_ get EINVAL from sem_post! This is fixed in newer glibc(2.21).
>>
>> Note that mutexes have had same issue on some platforms:
>> https://sourceware.org/bugzilla/show_bug.cgi?id=13690
>> Fixed in 2.23.
>>
>> Since we only have one handshake operation running at anytime (safepoints and 
>> handshakes are also mutual exclusive, both run on VM Thread) we can actually 
>> always use the same semaphore. This patch changes the _done semaphore to be 
>> static instead, thus avoiding the post<->destroy race.
>>
>> Patch also contains some small changes which remove of dead code, remove 
>> unneeded state, handling of cases which we can't easily say will never happen 
>> and some additional error checks.
>>
>> Handshakes test passes, but they don't trigger the original issue, so more 
>> interesting is that this issue do not happen when running ZGC which utilize 
>> handshakes with the static semaphore.
>>
>> Thanks, Robbin