Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Steven Rostedt (rostedt@goodmis.org) wrote:
> On Thu, 2010-01-07 at 12:58 -0800, Paul E. McKenney wrote:
> 
> > I believe that I am worried about a different scenario.  I do not believe
> > that the scenario you lay out above can actually happen.  The pair of
> > schedules on CPU 2 have to act as a full memory barrier, otherwise,
> > it would not be safe to resume a task on some other CPU.
> 
> I'm not so sure about that. The update of ->curr happens inside a
> spinlock, which is a rmb() ... wmb() pair. Must be, because a spin_lock
> must be an rmb otherwise the loads could move outside the lock, and the
> spin_unlock must be a wmb() otherwise what was written could move
> outside the lock.

Hrm, a rmb + wmb pair is different than a full mb(), because rmb and wmb
can be reordered ont wrt the other. The equivalence is more:

mb() = rmb() + sync_core() + wmb()

> 
> 
> >   If the pair
> > of schedules act as a full memory barrier, then the code in
> > synchronize_rcu() that looks at the RCU read-side state would see that
> > CPU 2 is in an RCU read-side critical section.
> > 
> > The scenario that I am (perhaps wrongly) concerned about is enabled by
> > the fact that URCU's rcu_read_lock() has a load, some checks, and a store.
> > It has compiler constraints, but no hardware memory barriers.  This
> > means that CPUs (even x86) can execute an rcu_dereference() before the
> > rcu_read_lock()'s store has executed.
> > 
> > Hacking your example above, keeping mind that x86 can reorder subsequent
> > loads to precede prior stores:
> > 
> > 
> > 	CPU 1				CPU 2
> >      -----------                    -------------
> > 
> > 	<user space>			<kernel space, switching to task>
> > 
> > 					->curr updated
> > 
> > 					<long code path, maybe mb?>
> > 
> > 					<user space>
> > 
> > 					rcu_read_lock(); [load only]
> > 
> > 					obj = list->next
> > 
> > 	list_del(obj)
> > 
> > 	sys_membarrier();
> > 	< kernel space >
> 
> Well, if we just grab the task_rq(task)->lock here, then we should be
> OK? We would guarantee that curr is either the task we want or not.
> 
[...]

Yes, I'll do some testing to figure out how much overhead this has.
Probably not much, but it's an iteration on all CPUs, so it will be a
bit larger on big iron. Clearly taking the run queue lock would be the
safest way to proceed.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
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