[Click] NotifierQueue on multi-threaded Click problem.

[Eddie Kohler]

AARGH!

Well, thanks as usual for the bug report.

I think though that this is one of those cases where your lock does not 
actually fix the problem, it just makes the race condition much less 
likely by slowing down the code.

I have attempted to fix the problem in a different way.  The commit 
message has more.  Let me know how it does....
Eddie




Joonwoo Park wrote:
> Unfortunately, A few hours ago I found that my test machine has gone
> into hibernation.
> It had worked for about 3 ~ 4 days maybe.
> And I found that I can reproduce the problem in short time with
> SLEEPINESS_TRIGGER=1.
> IMHO, I think process_pending() still has problem.
> When another thread's Task::add_pending see 'if (!_pending_nextptr)'
> as FALSE, at the same time if we set it to 0, thread->add_pending()
> cannot be called again (only if _pending_reschedule is FALSE).
> As a result, It's seems that a routerthread go sleep forever.
> I tried patch and It works fine although SLEEPINESS_TRIGGER=1. (I have
> never seen before working SLEEPINESS_TRIGGER=1)
> How about this?
> 
> -
> Signed-off-by: Joonwoo Park <joonwpark81 at gmail.com>
> 
>  lib/master.cc |    8 ++++++++
>  1 files changed, 8 insertions(+), 0 deletions(-)
> 
> diff --git a/lib/master.cc b/lib/master.cc
> index 5a56371..cef62be 100644
> --- a/lib/master.cc
> +++ b/lib/master.cc
> @@ -370,7 +370,15 @@ Master::process_pending(RouterThread *thread)
>      // process the list
>      while (Task *t = Task::pending_to_task(my_pending)) {
>  	my_pending = t->_pending_nextptr;
> +	// when another thread's Task::add_pending see 'if (!_pending_nextptr)'
> +	// as FALSE, at the same time if we set it to 0, thread->add_pending()
> +	// cannot be called again (only if _pending_reschedule is 0).
> +	bool lock = !t->_pending_reschedule;
> +	if (lock)
> +		flags = _task_lock.acquire();
>  	t->_pending_nextptr = 0;
> +	if (lock)
> +		_task_lock.release(flags);
>  	t->process_pending(thread);
>      }
>  }
> -
> 
> Joonwoo Park