RFR(m): 8221734: Deoptimize with handshakes

Daniel D. Daugherty daniel.daugherty at oracle.com
Mon May 20 22:33:09 UTC 2019

> This full inc from v2 (review + stress test):
> http://cr.openjdk.java.net/~rehn/8221734/v3/inc/ 

     No comments.

     No comments.

     L1145: // Deoptimize all(most) methods
         Yea... that comment and the name of the function need help even
         before your changes. Sigh... Maybe:

            // Mark methods for deopt (if safe or possible).

     L1151:     // Not installed are unsafe to mark for deopt, normally 
never deopted.
     L1152:     // A not_entrant method may become a zombie at any time,
     L1153:     // since we don't know on which side of last safepoint 
it became not_entrant
     L1154:     // (state must be in_use).
     L1155:     // Native method are unsafe to mark for deopt, normally 
never deopted.
     L1156:     if (!nm->method()->is_method_handle_intrinsic() &&
     L1157:         !nm->is_not_installed() &&
     L1158:         nm->is_in_use() &&
     L1159:         !nm->is_native_method()) {
     L1160:       nm->mark_for_deoptimization();
         Please consider replacing L1151-5 with the following comment after
         L1159. I prefer the comment for an if-statement to be inside the
         if-statement (but this is Compiler team code so they may have a
         different preference).

                  // Intrinsics and native methods are never deopted. A 
method that is
                  // not installed yet or is not in use is not safe to 
deopt; the
                  // is_in_use() check covers the not_entrant and not 
zombie cases.
                  // Note: A not_entrant method can become a zombie at 
anytime if it was
                  // made not_entrant before the previous 

     L1187:     // only_alive_and_not_unloading returns not_entrant 
     L1188:     // A not_entrant can become a zombie at anytime,
     L1189:     // if it was made not_entrant before previous 
     L1190:     // We check that it is not not_entrant and not zombie,
     L1191:     // by checking is_in_use().
     L1192:     if (nm->is_marked_for_deoptimization() && nm->is_in_use()) {
     L1193:       nm->make_not_entrant();
         Please consider replacing L1187-91 with the following comment after
         L1192 (also indented):
                  // only_alive_and_not_unloading() can return 
not_entrant nmethods.
                  // A not_entrant method can become a zombie at anytime 
if it was
                  // made not_entrant before the previous 
safepoint/handshake. The
                  // is_in_use() check covers the not_entrant and not 
zombie cases
                  // that have become true after the method was marked 
for deopt.

     No comments.

     No comments.

     L957:   // We need to check if both the _code and 
     L958:   // refer to this nmethod because there is a race in setting 
these two fields
     L959:   // in Method* as seen in bugid 4947125.
     L960:   // If the vep() points to the zombie nmethod, the memory 
for the nmethod
     L961:   // could be flushed and the compiler and vtable stubs could 
still call
     L962:   // through it.
     L963:   if (code() == compare ||
     L964:       from_compiled_entry() == compare->verified_entry_point()) {
         Now that you've moved the comment closer to the code, I can see a
         disconnect between the comment and the code. The comment:

             // ... check if both the _code and 
             // refer to this nmethod

         The code:

             code() == compare ||
             from_compiled_entry() == compare->verified_entry_point()

         So the comment is "both" "and" and the the code is "||". One of
         these is not right.

     No comments.

     No comments.

     No comments.

     No comments.

     L1297: void Deoptimization::revoke_safepoint(JavaThread* thread, 
frame fr, RegisterMap* map) {
         Perhaps revoke_using_safepoint().

     L1311: void Deoptimization::revoke_handshake(JavaThread* thread, 
frame fr, RegisterMap* map) {
         Perhaps revoke_using_handshake().

     old L1325:     ObjectSynchronizer::inflate(thread, obj, 
         Why was this deleted?
         Update: See synchronizer.cpp below.

     No comments.

     Deleting ObjectSynchronizer::inflate() in 
     which is now Deoptimization::revoke_handshake() allows these changes
     to be reverted...

     L2:  * Copyright (c) 2014, 2016, Oracle and/or its affiliates. All 
rights reserved.
         This is listed as a new file, but it has an old (and dual) 
copyright year.

     Do any of the options used in the test require non-product bits?
     (I don't think so, but...)


On 5/15/19 2:26 AM, Robbin Ehn wrote:
> Hi, please see this update.
> I think I got all review comments fix.
> Long story short, I was concerned about test coverage, so I added a 
> stress test
> using the WB, which sometimes crashed in rubbish code.
> There are two bugs in the methods used by WB_DeoptimizeAll.
> (Seems I'm the first user)
> CodeCache::mark_all_nmethods_for_deoptimization();
> When iterating the nmethods we could see the methods being create in:
> void AdapterHandlerLibrary::create_native_wrapper(const methodHandle& 
> method)
> And deopt the method when it was in use or before.
> Native wrappers are suppose to live as long as the class.
> I filtered out not_installed and native methods.
> Deoptimization::deoptimize_all_marked();
> The issue is that a not_entrant method can go to zombie at anytime.
> There are several ways to make a nmethod not go zombie: nmethodLocker, 
> have it
> on stack, avoid safepoint poll in some states, etc.., which is also 
> depending on
> what type of nmethod.
> The iterator only_alive_and_not_unloading returns not_entrant 
> nmethods, but we
> don't know there state prior last poll.
> in_use -> not_entrant -> #poll# -> not_entrant -> zombie
> If the iterator returns the nmethod after we passed the poll it can 
> still be
> not_entrant but go zombie.
> The problem happens when a second thread marks a method for deopt and 
> makes it
> not_entrant. Then after a poll we end-up in deoptimize_all_marked(), 
> but the
> method is not yet a zombie, so the iterator returns it, it becomes a 
> zombie thus
> pass the if check and later hit the assert.
> So there is a race between the iterator check of state and 
> if-statement check of
> state. Fixed by also filtering out zombies.
> If the stress test with correction of the bugs causes trouble in 
> review, I can
> do a follow-up with the stress test separately.
> Good news, no issues found with deopt with handshakes.
> This is v3:
> http://cr.openjdk.java.net/~rehn/8221734/v3/webrev/
> This full inc from v2 (review + stress test):
> http://cr.openjdk.java.net/~rehn/8221734/v3/inc/
> This inc is the review part from v2:
> http://cr.openjdk.java.net/~rehn/8221734/v3/inc_review/
> This inc is the additional stress test with bug fixes:
> http://cr.openjdk.java.net/~rehn/8221734/v3/inc_test/
> Additional biased locking change:
> The original code use same copy of markOop in revoke_and_rebias.
> The keep same behavior I now pass in that copy into fast_revoke.
> The stress test passes hundreds of iterations in mach5.
> Thousands stress tests locally, the issues above was reproduce-able.
> Inc changes also passes t1-5.
> As usual with this change-set, I'm continuously running more test.
> Thanks, Robbin
> On 2019-04-25 14:05, Robbin Ehn wrote:
>> Hi all, please review.
>> Let's deopt with handshakes.
>> Removed VM op Deoptimize, instead we handshake.
>> Locks needs to be inflate since we are not in a safepoint.
>> Goes on top of:
>> https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2019-April/033491.html 
>> Code:
>> http://cr.openjdk.java.net/~rehn/8221734/v1/webrev/index.html
>> Issue:
>> https://bugs.openjdk.java.net/browse/JDK-8221734
>> Passes t1-7 and multiple t1-5 runs.
>> A few startup benchmark see a small speedup.
>> Thanks, Robbin

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