RFR: 8266742: Check W^X state on possible safepoint [v2]

[Anton Kozlov]

On Mon, 17 May 2021 07:01:52 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> src/hotspot/share/runtime/safepointMechanism.inline.hpp line 67:
>> 
>>> 65: }
>>> 66: 
>>> 67: void SafepointMechanism::process_if_requested(JavaThread* thread) {
>> 
>> Looking at the stack from the crash:
>> 
>> 
>> V [libjvm.dylib+0x8c1060] _ZN21MarkActivationClosure12do_code_blobEP8CodeBlob+0x3c 
>> V [libjvm.dylib+0x8fea78] _ZN10JavaThread11nmethods_doEP15CodeBlobClosure+0x84 
>> V [libjvm.dylib+0x401100] _ZN14HandshakeState18process_self_innerEv+0x18c 
>> V [libjvm.dylib+0x400f60] _ZN14HandshakeState15process_by_selfEv+0x48 
>> V [libjvm.dylib+0x832690] _ZN18SafepointMechanism25process_if_requested_slowEP10JavaThread+0x54 
>> V [libjvm.dylib+0x8fe4b8] _ZN10JavaThread44check_safepoint_and_suspend_for_native_transEPS_+0x24 
>> V [libjvm.dylib+0x92395c] _ZN19ProgrammableInvoker13invoke_nativeEPFvPhES0_P10JavaThread+0x118 
>> V [libjvm.dylib+0x923aa8] PI_invokeNative+0x104 
>> J 2565 jdk.internal.foreign.abi.ProgrammableInvoker.invokeNative(JJ)V jdk.incubator.foreign at 17-ea (0 bytes) @ 0x00000001189eeed0 
>> 
>> 
>> why aren't we adding the assertion closer to the frame that crashed, where we need it, like for example:
>> 
>> `HandshakeState::process_self_inner()`
>
> If we push the check too low we only do it when we hit this code and there is something to process, but we want to detect when we reach this code in the wrong state even if we wouldn't actually crash at this time due to there not being an active poll.
> 
> David

David, thanks, this is exactly how it meant to be.

The crash happens when a thread is synchronized for MarkActivationClosure. Having the check in the HandshakeState::process_self_inner would catch the wrong state when a thread is synchronized also for something else. The patch catches the wrong state if a thread polls synchronization, it does not need to actually synchronize to trigger the check. So we'll catch more.

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PR: https://git.openjdk.java.net/jdk/pull/3920