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

Erik Österlund erik.osterlund at oracle.com
Mon Jun 18 13:12:01 UTC 2018

Hi Robbin,

Looks good.


On 2018-06-18 15:07, Robbin Ehn wrote:
> Hi all,
> After some internal discussions I changed the patch to:
> http://rehn-ws.se.oracle.com/cr_mirror/8204166/v2/
> 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

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