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Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
 barrier
From: Steven Rostedt <rostedt@goodmis.org>
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On Thu, 2010-01-07 at 11:16 -0800, Paul E. McKenney wrote:

> > Note, we are not suggesting optimizations. It has nothing to do with
> > performance of the syscall. We just can't allow one process to be DoSing
> > another process on another cpu by it sending out millions of IPIs.
> > Mathieu already showed that you could cause a 2x slowdown to the
> > unrelated tasks.
> 
> I would have said that we are trying to optimize our way out of a DoS
> situation, but point taken.  Whatever we choose to call it, the discussion
> is on the suggested modifications, not strictly on the original patch.  ;-)

OK, I just want to get a better understanding of what can go wrong. A
sys_membarrier() is used as follows, correct? (using a list example)

	<user space>

	list_del(obj);

	synchronize_rcu();  -> calls sys_membarrier();

	free(obj);


And we need to protect against:

	<user space>

	read_rcu_lock();

	obj = list->next;

	use_object(obj);

	read_rcu_unlock();

where we want to make sure that the synchronize_rcu() makes sure that we
have passed the grace period of all takers of read_rcu_lock(). Now I
have not looked at the code that implements userspace rcu, so I'm making
a lot of assumptions here. But the problem that we need to avoid is:


	CPU 1				CPU 2
     -----------                    -------------

	<user space>			<user space>

					rcu_read_lock();

					obj = list->next

	list_del(obj)

					< Interrupt >
					< kernel space>
					<schedule>

					<kernel_thread>

					<schedule>

					< back to original task >

	sys_membarrier();
	< kernel space >

	if (task_rq(task)->curr != task)
	< but still sees kernel thread >

	< user space >

	< misses that we are still in rcu section >

	free(obj);

					< user space >

					use_object(obj); <=== crash!



I guess what I'm trying to do here is to understand what can go wrong,
and then when we understand the issues, we can find a solution.

-- Steve