8355627: Don't use ThreadCritical for EventLog list by coleenp · Pull Request #24954 · openjdk/jdk

coleenp · 2025-04-29T19:16:48Z

Use LockFreeStack to link events on the eventLog queue. They are never popped so this requires no further synchronization.
Tested by tier1-4.

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Issue

JDK-8355627: Don't use ThreadCritical for EventLog list (Bug - P4)

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Aleksey Shipilev (@shipilev - Reviewer)
Leonid Mesnik (@lmesnik - Reviewer)
Zhengyu Gu (@zhengyu123 - Reviewer)

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bridgekeeper · 2025-04-29T19:17:25Z

👋 Welcome back coleenp! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2025-04-29T19:18:38Z

@coleenp This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8355627: Don't use ThreadCritical for EventLog list

Reviewed-by: shade, lmesnik, zgu

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At the time when this comment was updated there had been 168 new commits pushed to the master branch:

7b31762: 8354235: Test javax/net/ssl/SSLSocket/Tls13PacketSize.java failed with java.net.SocketException: An established connection was aborted by the software in your host machine
0cd0afb: 8355913: RISC-V: improve hotspot/jtreg/compiler/vectorization/runner/BasicFloatOpTest.java
0a697f6: 8344708: Implement JEP 511: Module Import Declarations
... and 165 more: https://git.openjdk.org/jdk/compare/f98af0ad617a445362859e58af48258bfd5bed03...master

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openjdk · 2025-04-29T19:19:15Z

@coleenp The following label will be automatically applied to this pull request:

hotspot

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

mlbridge · 2025-04-29T19:21:58Z

Webrevs

shipilev

Not having extra locks during initialization is nice. But I am somewhat confused why we need LockFreeStack here. Looks like we "only" need to build the linked list of EventLog-s in thread-safe manner? So why don't just do our usual Atomic dance, e.g. (untested!):

EventLog::EventLog() {
  EventLog* old_head;
  do {
    old_head = Atomic::load(&Events::_logs);
    _next = old_head;
  } while (Atomic::cmpxchg(&Events::_logs, old_head, this, memory_order_relaxed) != old_head);
}

coleenp · 2025-04-30T11:53:10Z

So I was playing around with LockFreeStack because I thought it might be a good way to avoid writing yet another CAS loop in the code. But you're right, I don't really use LockFreeStack which isn't so lock free since it requires synchronization to pop. I couldn't decide whether I wanted to use it partially or write the CAS loop. I take it this is a vote for the CAS loop.

zhengyu123 · 2025-04-30T20:16:18Z

src/hotspot/share/utilities/events.cpp

+  do {
+    old_head = Atomic::load(&Events::_logs);
+    _next = old_head;
+  } while (Atomic::cmpxchg(&Events::_logs, old_head, this, memory_order_relaxed) != old_head);


I don't think it is safe to use memory_order_relaxed order if events are added after JVM enters multi-thread mode.

Thanks for this. I didn't know why Aleksey suggested it. I'll remove it.

Maybe @shipilev meant memory_order_release? Anyway, I guess we don't need to optimize it.

I saw no point in enforcing memory ordering mode here, as it looks like we only did ThreadCritical for mutual exclusion. Note that we do not have a matching acquire on list traversals, so seqcst/release on list additions would be incomplete. That only reinforces my original thinking: we are riding on memory ordering given by something else, I'd guess the initialization sequence itself.

But I won't quibble, it is a very minor optimization.

I guess we rely on memory ordering by address dependency on the reader's side.

First of all, nothing on the writer's side can fix the reader's side.

We're basically relying on volatile load + dependency chain on the reader's side. This is very likely to work, but C++ memory model specialists may find this questionable.

Are we on the same page? which load you are talking about?

I'm talking about Atomic::load() which is essentially a volatile load without ordering.

I don't believe we need reader side barrier, given the conservative order on writer side. @shipilev any opinion?

Well, if we are going fully pedantic here, we only need to make sure that the EventLog::_next is properly visible to the code that traverses the EventLog list. This realistically only matters if there are no synchronization points in between those, which I suspect are plenty. E.g. this is the same thing that make all other init code robust in practice: things that happen at init are good to go.

If you want to cover the path to be fully 100% bullet-proof and squeeze some performance at the same time, then I think cmpxchg to Events::_logs should be release and loads from there should be acquire-s.

lmesnik

The changes looks good.

shipilev

Looks fine, thanks.

zhengyu123

LGTM

coleenp · 2025-05-02T11:39:23Z

Thank you for reviewing and commenting, Aleksey, Zhengyu, Leonid and Martin.
/integrate

openjdk · 2025-05-02T11:40:22Z

Going to push as commit afb9134.
Since your change was applied there have been 184 commits pushed to the master branch:

811f117: 8355980: RISC-V: remove vmclr_m before vmsXX and vmfXX
d29700c: 8344706: Implement JEP 512: Compact Source Files and Instance Main Methods
bd7c778: 8350549: MethodHandleProxies.WRAPPER_TYPES is not thread-safe
... and 181 more: https://git.openjdk.org/jdk/compare/f98af0ad617a445362859e58af48258bfd5bed03...master

Your commit was automatically rebased without conflicts.

openjdk · 2025-05-02T11:40:30Z

@coleenp Pushed as commit afb9134.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

coleenp added 2 commits April 25, 2025 17:30

8355627: Don't use ThreadCritical for EventLog list

3792660

Fix comment.

f4c9611

openjdk bot added the rfr Pull request is ready for review label Apr 29, 2025

openjdk bot added the hotspot hotspot-dev@openjdk.org label Apr 29, 2025

shipilev reviewed Apr 30, 2025

View reviewed changes

Use CAS loop instead of lockFreeStack.

0930c85

zhengyu123 reviewed Apr 30, 2025

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coleenp added 2 commits April 30, 2025 20:32

cleanups

d89df3e

remove memory order specification

da5c2a7

lmesnik approved these changes Apr 30, 2025

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openjdk bot added the ready Pull request is ready to be integrated label Apr 30, 2025

shipilev approved these changes May 1, 2025

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zhengyu123 approved these changes May 1, 2025

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openjdk bot added the integrated Pull request has been integrated label May 2, 2025

openjdk bot closed this May 2, 2025

openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels May 2, 2025

coleenp deleted the tc branch May 2, 2025 12:42

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