I'll file a follow up bug after the dust settles for 8227117.
I filed the following:
JDK-8227338 templateInterpreter.cpp: copy_table() needs to be safer
https://bugs.openjdk.java.net/browse/JDK-8227338
Dan
On 7/5/19 1:07 PM, Daniel D. Daugherty wrote:
On 7/4/19 3:10 AM, Erik Österlund wrote:
Hi Dan,
Thanks for picking this up. The change looks good.
Thanks! Of course, just the size of the comment below makes me wonder
what I got myself into... :-) And I was so happy that the non-logging
part of the fix was an else-statement with _one_ line...
However, when reviewing this, I looked at the code for actually
restoring the table (ignore/notice safepoints). It copies the
dispatch table for the interpreter. There is a comment stating it is
important the copying is atomic for MT-safety, and I can definitely
see why. However, the copying the line after that comment is in fact
not atomic.
Actually, the comment doesn't mention 'atomic', but that's probably
because the code and the comment are very, very old. It mentions
'word wise for MT safety' and I agree that 'atomic' is what the
person likely meant...
The history:
$ sgv src/share/vm/interpreter/templateInterpreter.cpp | grep 'The
copy has to occur word wise for MT safety'
1.1 // Copy non-overlapping tables. The copy has to occur word
wise for MT safety.
$ sp -r1.1 src/share/vm/interpreter/templateInterpreter.cpp
src/share/vm/interpreter/SCCS/s.templateInterpreter.cpp:
D 1.1 07/08/29 13:42:26 sgoldman 1 0 00600/00000/00000
MRs:
COMMENTS:
6571248 - continuation_for is specialized for template interpreter
Hmmm... I expected that comment to be even older... ahhhh... a little
more poking around and I found:
$ sgv -r1.147 src/share/vm/interpreter/interpreter.cpp | grep 'The
copy has to occur word wise for MT safety'
1.147 // Copy non-overlapping tables. The copy has to occur word
wise for MT safety.
$ sp -r1.147 src/share/vm/interpreter/interpreter.cpp
src/share/vm/interpreter/SCCS/s.interpreter.cpp:
D 1.147 99/02/17 10:14:36 steffen 235 233 00008/00002/00762
MRs:
COMMENTS:
This makes more sense (timeline wise) and dates back to when all
of the interpreter was in vm/interpreter/interpreter.cpp.
Here is the copying code in templateInterpreter.cpp:
static inline void copy_table(address* from, address* to, int size) {
// Copy non-overlapping tables. The copy has to occur word wise for
MT safety.
while (size-- > 0) *to++ = *from++;
}
Copying using a loop of non-volatile loads and stores can and
definitely will on some compilers turn into memcpy calls instead as
the compiler (correctly) considers that an equivalent transformation.
Yet another C++ compiler optimization land mine... sigh...
And memcpy does not guarantee atomicity. Indeed on some platforms it
is not atomic. On some platforms it will even enjoy out-of-thin-air
values.
That last bit is scary...
Perhaps Copy::disjoint_words_atomic() would be a better choice for
atomic word copying. If not, at the very least we should use
Atomic::load/store here.
Copy::disjoint_words_atomic() sounds appealing...
For those folks that aren't familiar with this part of safepointing...
SafepointSynchronize::arm_safepoint() calls
Interpreter::notice_safepoints()
which calls calls copy_table(). So we're not at a safepoint yet, and,
in fact,
we're trying to bring those pesky JavaThreads to a safepoint...
SafepointSynchronize::disarm_safepoint() calls
Interpreter::ignore_safepoints()
which also calls copy_table(). However, we did that before we have
woken the
JavaThreads that are blocked for the safepoint so that use of
copy_table is safe:
// Release threads lock, so threads can be created/destroyed again.
Threads_lock->unlock();
// Wake threads after local state is correctly set.
_wait_barrier->disarm();
}
The 'Threads_lock->unlock()' should synchronize memory so that the
restored
table should be properly synced out to memory...
Having said that, the fix for that issue seems like a separate RFE,
because it has been sitting there for a lot longer than TLH has been
around.
Yes I would like to keep the copy_table() issue for a separate bug
(not RFE).
I'll file a follow up bug after the dust settles for 8227117.
Thanks again for the review!
Dan
Thanks,
/Erik
On 2019-07-04 04:04, Daniel D. Daugherty wrote:
Greetings,
Robbin recently discovered this issue with Thread Local Handshakes.
Since
he's not available at the moment, I'm handling the issue:
JDK-8227117 normal interpreter table is not restored after
single stepping with TLH
https://bugs.openjdk.java.net/browse/JDK-8227117
When using Thread Local Handshakes, the normal interpreter table is
not restored after single stepping. This issue is caused by the
VM_ChangeSingleStep VM-op relying on SafepointSynchronize::end() to
restore the normal interpreter table for the "off" case.
Prior to Thread Local Handshakes, this was a valid assumption to make.
SafepointSynchronize::end() has been refactored into
disarm_safepoint() and it only calls Interpreter::ignore_safepoints()
on the global safepoint branch. That matches up with the call to
Interpreter::notice_safepoints() that is also on the global safepoint
branch.
The solution is for the VM_ChangeSingleStep VM-op for the "off" case
to call Interpreter::ignore_safepoints() directly.
Here's the webrev URL:
http://cr.openjdk.java.net/~dcubed/8227117-webrev/0_for_jdk14/
The fix is just a small addition to VM_ChangeSingleStep::doit():
if (_on) {
Interpreter::notice_safepoints();
+ } else {
+ Interpreter::ignore_safepoints();
}
Everything else is just new logging support for future debugging of
interpreter table management and single stepping.
Tested this fix with Mach5 Tier[1-3] on the standard Oracle platforms.
Mach5 Tier[4-6] on standard Oracle platforms is running now.
Thanks, in advance, for questions, comments or suggestions.
Dan
