On Thu, Feb 18, 2021 at 6:36 PM pat m <patmccl...@gmail.com> wrote:
>
> Hello,
>
> I am working on a Node.js module that creates a CodeEventHandler
> (https://v8docs.nodesource.com/node-14.15/d2/d08/classv8_1_1_code_event_handler.html).
> I've seen that it's Handle method can be called from outside the main
> isolate thread, i.e. for parallel garbage collection:
>
> #3 0x00007f393a5fb0f3 in
> codeevents::FnInspectCodeEventHandler::Handle(v8::CodeEvent*) () from
> /home/ec2-user/module.node
> #4 0x0000000000e43705 in
> v8::internal::ExternalCodeEventListener::CodeMoveEvent(v8::internal::AbstractCode,
> v8::internal::AbstractCode) ()
> #5 0x0000000000d5707a in
> v8::internal::ProfilingMigrationObserver::Move(v8::internal::AllocationSpace,
> v8::internal::HeapObject, v8::internal::HeapObject, int) ()
> #6 0x0000000000d6d41b in void
> v8::internal::EvacuateVisitorBase::RawMigrateObject<(v8::internal::EvacuateVisitorBase::MigrationMode)1>(v8::internal::EvacuateVisitorBase*,
> v8::internal::HeapObject, v8::internal::HeapObject, int,
> v8::internal::AllocationSpace) ()
> #7 0x0000000000d706df in
> v8::internal::FullEvacuator::RawEvacuatePage(v8::internal::MemoryChunk*,
> long*) ()
> #8 0x0000000000d5d17f in
> v8::internal::Evacuator::EvacuatePage(v8::internal::MemoryChunk*) ()
> #9 0x0000000000d5d67f in
> v8::internal::PageEvacuationTask::RunInParallel(v8::internal::ItemParallelJob::Task::Runner)
> ()
> #10 0x0000000000d4f8c5 in v8::internal::ItemParallelJob::Task::RunInternal()
> ()
> #11 0x0000000000c6c9eb in non-virtual thunk to
> v8::internal::CancelableTask::Run() ()
> #12 0x0000000000a71405 in node::(anonymous
> namespace)::PlatformWorkerThread(void*) ()
> #13 0x00007f3bd38d440b in start_thread () from /lib64/libpthread.so.0
> #14 0x00007f3bd360ef9f in clone () from /lib64/libc.so.6
>
> Is it safe to access the String values returned by GetFunctionName and
> GetScriptName in this context (i.e. using String.Write)? Any method to
> access the underlying characters requires passing the isolate as a parameter,
> and all of the docs I can find say it's not safe to use the isolate without
> entering it via Locker. However that isn't viable in this case as the
> Node.js main thread locks the isolate on startup, so trying to get a lock on
> the isolate simply deadlocks the thread.
>
> thanks
> pat
In the case of a CODE_MOVED event, event.script doesn't point to
anything. That field is only valid (and then only sometimes) for
CODE_ADDED events.
I noticed a small bug - V8 doesn't clear unused JitCodeEvent fields
when it emits a CODE_MOVED event - that might be why you get random
crashes instead of nullptr segfaults.
diff --git a/src/logging/log.cc b/src/logging/log.cc
index 7738cab831..94497f6502 100644
--- a/src/logging/log.cc
+++ b/src/logging/log.cc
@@ -751,6 +751,7 @@ void JitLogger::CodeMoveEvent(AbstractCode from,
AbstractCode to) {
base::MutexGuard guard(&logger_mutex_);
JitCodeEvent event;
+ memset(static_cast<void*>(&event), 0, sizeof(event));
event.type = JitCodeEvent::CODE_MOVED;
event.code_type =
from.IsCode() ? JitCodeEvent::JIT_CODE : JitCodeEvent::BYTE_CODE;
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