RFR: 8253833: mutexLocker assert_locked_or_safepoint should not access VMThread state from non-VM-thread

[Robbin Ehn]

On Thu, 8 Oct 2020 22:11:12 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> I was hope that "git blame" would help, but most of this code is ancient:
>> 
>> da18495f385b (Coleen Phillimore 2019-08-22 09:51:36 -0400 166) void assert_locked_or_safepoint(const Mutex* lock) {
>> 8153779ad32d (J. Duke           2007-12-01 00:00:00 +0000 167)   // check if this thread owns the lock (common case)
>> 8153779ad32d (J. Duke           2007-12-01 00:00:00 +0000 168)   assert(lock != NULL, "Need non-NULL lock");
>> 8153779ad32d (J. Duke           2007-12-01 00:00:00 +0000 169)   if (lock->owned_by_self()) return;
>> 8153779ad32d (J. Duke           2007-12-01 00:00:00 +0000 170)   if (SafepointSynchronize::is_at_safepoint()) return;
>> 8153779ad32d (J. Duke           2007-12-01 00:00:00 +0000 171)   if (!Universe::is_fully_initialized()) return;
>> 8153779ad32d (J. Duke           2007-12-01 00:00:00 +0000 172)   // see if invoker of VM operation owns it
>> 8153779ad32d (J. Duke           2007-12-01 00:00:00 +0000 173)   VM_Operation* op = VMThread::vm_operation();
>> 8153779ad32d (J. Duke           2007-12-01 00:00:00 +0000 174)   if (op != NULL && op->calling_thread() ==
>> lock->owner()) return; 1e71f6773659 (David Lindholm    2015-09-29 11:02:08 +0200 175)   fatal("must own lock %s",
>> lock->name()); 8153779ad32d (J. Duke           2007-12-01 00:00:00 +0000 176) }
>> 
>> Update: Stripped the filename from the "git blame" output so it wasn't quite so wide...
>> Update: Changing to a `code` quote helped the formatting a bit...
>
> The initialization bail-out was added "1.86 99/02/19 17:05:07", but no comments or associated bug report.

Universe is initialized before VM thread is ready to process operations.
So while that bail-out is true, there can never be any vm ops.

I believe instead of:
`if (!Universe::is_fully_initialized()) return;`
We really want:
`if (Threads::number_of_threads() == 0) return;`
But I'm not sure and totally out of scope.

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PR: https://git.openjdk.java.net/jdk/pull/563