| Deadlock between Job and event listener on Display thread [message #1843245] |
Wed, 21 July 2021 11:18  |
 Elie Richa
Messages: 72
Registered: February 2016 |
Member |
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Hello folks,
In the Eclipse integration of my language I set up an editor callback to listen to keystrokes and determine the semantic object where they occurred. I also have Eclipse Jobs that can simultaneously be trying to modify the document. I am reaching a dead-lock and I'm not sure how to resolve it.
The Job is something like this:
val editor = HandlerUtil.getActiveEditor(event) as XtextEditor
val doc = editor.document
val job = Job.create("Modifying document") [
doc.modify [
// Modify the semantic model
]
return null
]
job.schedule
On the other hand I setup an SWT VerifyListener to intercept keystrokes in this way:
(Omitting a lot of injection and initialization code. Assume everything is working correctly with no runtime exceptions. The dead-lock is the only issue)
class BLEditorCallback extends IXtextEditorCallback.NullImpl {
override afterCreatePartControl(XtextEditor editor) {
super.afterCreatePartControl(editor)
partVerifyListener = new VerifyListener() {
override verifyText(VerifyEvent e) {
val selProv = editor.selectionProvider.selection
val currentObj = if (selProv instanceof ITextSelection) {
val offset = selProv.offset
editor.document.readOnly [
objAtOffsetHelper.resolveContainedElementAt(it, offset)
]
}
}
}
editor.addVerifyListener(partVerifyListener)
}
}
When I launch the job, and then hit a key in the editor while the job is running, I get the dead-lock. The Job thread and the Display thread stacks look like this:
"Worker-3: Modifying document" #44 prio=5 os_prio=31 cpu=15193.22ms elapsed=443.31s tid=0x00007fc49a827000 nid=0x16603 waiting on condition [0x000070000c083000]
java.lang.Thread.State: TIMED_WAITING (parking)
at jdk.internal.misc.Unsafe.park(java.base@11.0.11/Native Method)
- parking to wait for <0x000000075e00a9b0> (a java.util.concurrent.Semaphore$NonfairSync)
at java.util.concurrent.locks.LockSupport.parkNanos(java.base@11.0.11/LockSupport.java:234)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedNanos(java.base@11.0.11/AbstractQueuedSynchronizer.java:1079)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.tryAcquireSharedNanos(java.base@11.0.11/AbstractQueuedSynchronizer.java:1369)
at java.util.concurrent.Semaphore.tryAcquire(java.base@11.0.11/Semaphore.java:415)
at org.eclipse.ui.internal.PendingSyncExec.acquire(PendingSyncExec.java:39)
at org.eclipse.ui.internal.PendingSyncExec.waitUntilExecuted(PendingSyncExec.java:88)
at org.eclipse.ui.internal.UISynchronizer.syncExec(UISynchronizer.java:142)
at org.eclipse.swt.widgets.Display.syncExec(Display.java:5048)
at org.eclipse.xtext.ui.util.DisplayRunnable.syncExec(DisplayRunnable.java:22)
at org.eclipse.xtext.ui.editor.model.XtextDocument.fireRewriteSessionChanged(XtextDocument.java:741)
at org.eclipse.jface.text.AbstractDocument.startRewriteSession(AbstractDocument.java:1456)
at org.eclipse.jface.text.RewriteSessionEditProcessor.performEdits(RewriteSessionEditProcessor.java:97)
at org.eclipse.xtext.ui.editor.model.edit.ReconcilingUnitOfWork.exec(ReconcilingUnitOfWork.java:63)
at org.eclipse.xtext.ui.editor.model.edit.ReconcilingUnitOfWork.exec(ReconcilingUnitOfWork.java:1)
at org.eclipse.xtext.resource.OutdatedStateManager.exec(OutdatedStateManager.java:70)
at org.eclipse.xtext.ui.editor.model.XtextDocument$XtextDocumentLocker.modify(XtextDocument.java:432)
- locked <0x000000075b0955f8> (a org.eclipse.xtext.resource.DerivedStateAwareResource)
at org.eclipse.xtext.ui.editor.model.XtextDocument.internalModify(XtextDocument.java:165)
at org.eclipse.xtext.ui.editor.model.XtextDocument.modify(XtextDocument.java:158)
at org.irit.xtext.blocklibrary.ui.commands.CreateModelTemplateForTestCaseHandler$3.run(CreateModelTemplateForTestCaseHandler.java:89)
at org.eclipse.core.runtime.jobs.Job$2.run(Job.java:187)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:63)
"main" #1 prio=6 os_prio=31 cpu=70760.75ms elapsed=460.51s tid=0x00007fc495049800 nid=0x307 waiting for monitor entry [0x00007ffeea45a000]
java.lang.Thread.State: BLOCKED (on object monitor)
at org.eclipse.xtext.ui.editor.model.XtextDocument$XtextDocumentLocker.internalReadOnly(XtextDocument.java:513)
- waiting to lock <0x000000075b0955f8> (a org.eclipse.xtext.resource.DerivedStateAwareResource)
at org.eclipse.xtext.ui.editor.model.XtextDocument$XtextDocumentLocker.priorityReadOnly(XtextDocument.java:489)
at org.eclipse.xtext.ui.editor.model.XtextDocument.priorityReadOnly(XtextDocument.java:145)
at org.irit.xtext.blocklibrary.ui.editor.BLBaselineProtectionListener.verifyText(BLBaselineProtectionListener.java:215)
at org.irit.xtext.blocklibrary.ui.editor.BLEditorCallback$1.verifyText(BLEditorCallback.java:57)
at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:277)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:89)
at org.eclipse.swt.widgets.Display.sendEvent(Display.java:4441)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1512)
The Job thread owns the DerivedStateAwareResource lock and is waiting to acquire the Display lock, while the Display thread owns the Display lock and is waiting to acquire the DerivedStateAwareResource. It's the typical scenario when two threads acquire a sequence of locks in a different order.
On the Display thread we can't change the order of locking because the Display lock is necessarily already taken before the VerifyEvent is triggered.
On the Job thread we can't change the order of locking because we don't want to acquire the Display lock for the entirety of the document modification (it takes some time).
I saw that if I use a CancelableUnitOfWork on the Job side, I could use doc.priorityReadOnly() on the Display thread to cause the cancellation of the Job. However I don't think that will solve it because the stack trace shows that the Job thread is blocked during AbstractDocument.startRewriteSession() so I won't have the opportunity of checking a cancel indicator.
A last solution I'm considering is to do something like this in the event listener:
partVerifyListener = new VerifyListener() {
override verifyText(VerifyEvent e) {
if (editor.document.hasModifyingUnitsOfWork()) {
// calling editor.document.readOnly() might cause a dead lock so
// display a dialog and reject the keystroke
e.doit = false
return;
}
val selProv = editor.selectionProvider.selection
val currentObj = if (selProv instanceof ITextSelection) {
val offset = selProv.offset
editor.document.readOnly [
objAtOffsetHelper.resolveContainedElementAt(it, offset)
]
}
}
}
And I could subclass XTextDocument and override modify() to track when modification jobs are ongoing.
Thoughts? Ideas?
Thanks in advance!
Elie Richa, Ph.D
Software Engineer, AdaCore
https://www.adacore.com
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