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From "Evgueni Brevnov" <evgueni.brev...@gmail.com>
Subject Re: [drlvm][threading] Thread.suspend() causes thread_native_fat_monitor.cpp::monitor_wait_impl() to give up the monitor
Date Tue, 05 Dec 2006 13:23:31 GMT
On 12/5/06, Nikolay Kuznetsov <nikolay.kuznetsov@gmail.com> wrote:
> Evgueni, Weldon,
>
> 1) NO_COND_VARS code will go to the trash soon, see HARMONY-2269 for
> details, Salikh has already prepared patches for this.
>
> 2) I agree, that this code is looks like (and there is) a workaround
> to let jdwp agent proceed with its usual scenarios for breakpoints. I
> will prepare testcases soon today, but in short the problem here is
> the following:
>
> 1. Thread T1 is waiting on monitor M1.
> 2. Thread T2 calls suspend on thread T1.
> 3. Thread T2 calls notify on  M1
> 4. Thread T1 finishes waiting, acquires M1 and falls into safepoint
> 5. Thread T2 calls monitor enter (M1) and Blocks on M1 (deadlock)

Nikolay,

I tend to think that the real problem is in the above scenario. T2
suspends T1 advisedly. So T2 should be ready that T1 may block while
holding all its monitors not only M1. (for example T1 may hold M2 as
well). If T2 tries to get M2 it will block on M2.....

To have more complete understanding what is going in here I need one
more piece of information. To be able to perform step 3 the T2 must
own M1. Could you show where T2 acquires M1 and where it releases it?

Thanks
Evgueni


>
> The sequence above is how jdwp agent implement breakpoints. The
> workaround we discussing preventing suspended thread from acquiring
> monitor (point 4).
>
> I'm going to write several testcases on this and will file an issue if
> we fine more elegant solution (suggestions are welcome).
>
> PS
>
> >What if suspend is requested right after "if (self->suspend_request)
> {..." statement?
> The question here is how to behave if we were suspended and notified
> after that but if we were first notified and then suspended it's out
> of this case.
>
>
> On 12/5/06, Evgueni Brevnov <evgueni.brevnov@gmail.com> wrote:
> > Weldon, Nikolay,
> >
> > I can't grok how Thread.suspend() interact with
> > thread_native_fat_monitor.cpp::monitor_wait_impl(). Could you explain
> > this?
> >
> > Here is my observations regarding monitor_wait_impl() implementation:
> >
> > 1) Line:197 discards previously set value of 'status' variable. What
> > if it is assigned to TM_ERROR_TIMEOUT value at line 193?
> > 2) The above trick with unlocking a mutex in case of 'suspend_request'
> > is not zero seems like a workaround to me. What if suspend is
> > requested right after "if (self->suspend_request) {..." statement?
> > 3) It was already discussed several times but I'll repeat it one more
> > time. It doesn't seem to be a solid (and safe) approach when a thread
> > with suspend state set to "disabled" calls monitor_wait_impl() and
> > thread's state is changed to "enabled" inside monitor_wait_impl().
> >
> > Evgueni
> >
> > On 12/4/06, Weldon Washburn <weldonwjw@gmail.com> wrote:
> > > The following code does not look right.  I looked in the JVM spec.  I can't
> > > find any mention that a Thread.suspend() should cause the target thread to
> > > give up any java lock.  I am trying to construct a test to confirm if this
> > > is a problem or not.  Anybody have any suggestions?
> > >
> > > line 216:
> > >
> > >
> > > if(self->suspend_request) {
> > >
> > >    hymutex_unlock(mon_ptr->mutex);
> > >
> > >    hythread_safe_point();
> > >
> > >    hymutex_lock(mon_ptr->mutex);
> > >
> > > }
> > >
> > >
> > > --
> > > Weldon Washburn
> > > Intel Enterprise Solutions Software Division
> > >
> > >
> >
>

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