CMS parallel initial mark; vm thread sneaking in Thread::try_lock()

[Thomas Schatzl]

On Thu, 2013-06-13 at 13:46 -0700, Hiroshi Yamauchi wrote:
>
>
> 
> On Thu, Jun 13, 2013 at 4:45 AM, Thomas Schatzl
> <thomas.schatzl at oracle.com> wrote:
> [... summary of the current state ...]
>
> Thomas, thanks for summarizing the discussion so far.
>
> I agree that if try_lock()/unlock() does what the manual/custom
> synchronization does (and as efficiently), it should suffice. No need
> to roll one's own.
>
> I also agree that adding the assert(!
> SafepointSynchronize::is_at_safepoint()) (or assert(!
> Thread->current()->is_VM_thread())) may be good enough and if sneaking
> can be turned off for a case like this, it'd be safer.

I think we should go for this solution, i.e. adding the assert (because
it would be wrong anyway to call this code during safepoint) and using
try_lock()/unlock().

Try_lock() itself is the same as the current code, and unlock() too
(except for some additional checks that should fail fast). The code that
is guarded handles occasional skipping already and actually exits fast
if the frequency is too high, so I do not see a big advantage doing
custom code.

Jon?

Thanks,
Thomas