On Thu, 21 Oct 2021 03:34:30 GMT, Chris Plummer <[email protected]> wrote:
>> You already release threadLock temporarily in `trackAppResume()` and you
>> used to release it temporarily in doPendingTasks(). I'm suggesting to
>> instead release it in `threadControl_onEventHandlerExit()` before calling
>> `doPendingTasks()` and then grab it again (after first grabbing the
>> handlerLock) in `doPendingTasks()` before calling `trackAppResume()`. What
>> can be modified in the ThreadNode during that time that you are concerned
>> about? threadLock would still be held when `blockOnDebuggerSuspend()` is
>> called. The only interesting lines that are currently executed while
>> holding the threadLock but no longer would be are:
>>
>> 2193 if (node->handlingAppResume) {
>> 2194 jthread resumer = node->thread;
>> 2195 jthread resumee = getResumee(resumer);
>>
>> I don't think any of those can change, because only the currently executing
>> thread can change them (when the Thread.resume() breakpoint is hit).
>>
>> The suspendCount is only meaningful when in `blockOnDebuggerSuspend()`, and
>> we'll be under the threadLock by that point.
>
> BTW, normally I would not suggest less locking simply because I *thought* it
> might not be needed. However, you already temporarily release and then regrab
> the lock. This means there is a period of time where the locking is not
> needed. What I've suggested is to better define (and expand) that period of
> time so the locking can be made cleaner. I think the actual expansion of time
> where the lock would no longer be held is pretty small, with a well defined
> set of code being executed that to me does not appear to require the lock,
> and if it did require the lock, then probably releasing the lock in the first
> place is a bug.
> move the locking and unlocking into doPendingTasks(). At the start of the
> node->handlingAppResume block, grab handlerLock and threadLock, and explain
> that handlerLock is being grabbed because trackAppResume() will (indirectly)
> try to grab it, and grabbing it before threadLock is necessary for lock
> ordering. Then grab threadLock and hold onto it until the end of the block.
So threadLock would be released at the end of the new block that depends on
node->handlingAppResume. After that block follow accesses to `pendingInterrupt`
and `pendingStop` that would be unsynchronized then. I think they do need to be
synchronized through threadLock though.
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PR: https://git.openjdk.java.net/jdk/pull/5849
