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From "Evgueni Brevnov" <evgueni.brev...@gmail.com>
Subject Re: [drlvm][threading] Is it safe to use hythread_suspend_all and hythread_resume_all?
Date Wed, 18 Oct 2006 07:22:06 GMT
On 10/17/06, Salikh Zakirov <Salikh.Zakirov@intel.com> wrote:
> Evgueni Brevnov wrote:
> > Hi All,
> >
> > I'd like to here you opinion regarding hythread_suspend_all and
> > hythread_resume_all functionality provided by TM. Actually I have to
> > separate questions:
> >
> > 1)  I found that hythread_suspend_all calls thread_safe_point_impl
> > inside. There is no assertion regarding thread's state upon entering
> > hythread_suspend_all. So it can be called in suspend disabled state
> > and nobody (at least me) expects to have a safe point during
> > hythread_suspend_all. The problem seems to be very similar with the
> > one discussed in "[drlvm][threading] Possible race condition in
> > implementation of conditional variables?" Your thoughts?
>
> The code you see is there to prevent following deadlock scenario:
>
>   Thread A           Thread B
>      |                  |
>      | <------------ suspend(A);
>      |           A->suspend_request = 1;
>      |        wait for A to reach a safepoint...
>      |                  |
>   suspend_all()         |
>  B->suspend_request = 1
> wait for B to reach a safepoint ...
>
> and then two threads are infinitely waiting one another.

Salikh, I see your scenario...I don't suggest to remove safe points
from hythread_suspend_all. Contrary I believe it makes sense to
suspend other threads only if it suspender thread is in a safe region.
Agree?

>
> > 2) Assume I need to suspend all threads in particular group. Ok I pass
> > that group to hythread_suspend_all. Later when all suspended threads
> > should be resumed I pass the same group to hythread_resume_all. But
> > threads were suspended group has changed. For example one new thread
> > was added to it. So the questions are. Is it acceptable to have such
> > "unsafe" functionality? Would it better to lock the group in
> > hythread_suspend_all and unlock it in hythread_resume_all.
>
> We may as well leave it as the responsibility of application / TI agent
> writer not to modify a suspended thread group.
> Why do you think this should be enforced?

In general, any "good" design should strive to eliminate/minimize
cases of illegal use of the interface. In other words it should be
hard to use it in a buggy way. In our case that means it is better to
ensure integrity inside TM instead of making application responsible
for that .... if we can do it inside what is the reason not to do it?
Moreover if you look to the spec of hythread_suspend_all it states
"...This method sets a suspend request for the every thread in the
group and then returns the iterator that can be used to traverse
through the suspended threads..." But implementation contradicts with
that. If group is changed while you are traversing through the group
you can get wrong thread. What is even worse you can get a crash when
one thread is iterating through the group while another thread inserts
new elements (or removes) to it.

Evgueni
>
>
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