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List:       openjdk-serviceability-dev
Subject:    Re: RFR: 8303086: SIGSEGV in JavaThread::is_interp_only_mode()
From:       Serguei Spitsyn <sspitsyn () openjdk ! org>
Date:       2023-06-30 21:01:09
Message-ID: q38LGFZj4NzHuclV1UlLu8MtO6PAa6wrSwjhj5UbxWU=.a102f924-cf4d-483a-b055-564b52fa3bca () github ! com
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On Fri, 30 Jun 2023 11:27:58 GMT, Serguei Spitsyn <sspitsyn@openjdk.org> wrote:

> The JVMTI function `SetEventNotificationMode` can set notification mode globally \
> (`event_thread == nullptr`) for all threads or for a specific thread (`event_thread \
> != nullptr`). To get a stable mount/unmount vision of virtual threads a \
> JvmtiVTMSTransitionDisabler helper object is created : `JvmtiVTMSTransitionDisabler \
> disabler(event_thread);` 
> In a case if `event_thread == nullptr` the VTMS transitions are disabled for all \
> virtual thread, otherwise they are disabled for a specific thread if it is virtual.
> The call to `JvmtiEventController::set_user_enabled()` makes a call to \
> `recompute_enabled()` at the end of its work to do a required bookkeeping. As part \
> of this work, the `recompute_thread_enabled(state)` is called for each thread from \
> the `ThreadsListHandle`, not only for the given `event_thread`: 
> ThreadsListHandle tlh;
> for (; state != nullptr; state = state->next()) {
> any_env_thread_enabled |= recompute_thread_enabled(state);
> }
> 
> This can cause crashes as VTMS transitions for other virtual threads are allowed.
> Crashes are observed in this small function:
> 
> bool is_interp_only_mode() {
> return _thread == nullptr ? _saved_interp_only_mode != 0 : \
> _thread->is_interp_only_mode(); }
> 
> In a case `_thread != nullptr` then the call needs to be executed: \
> `_thread->is_interp_only_mode()`. But the filed `_thread` can be already changed to \
> `nullptr` by a VTMS transition. 
> The fix is to always disable all transitions.
> Thanks to Dan and Patricio for great analysis of this crash!
> 
> Testing:
> - In progress: mach5 tiers 1-6

Patricio, Chris and Leonid, thank you for review.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/14728#issuecomment-1615188480


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