Re: Q: schedule() and implied barriers on arm64

[Paul E. McKenney]

On Fri, Oct 16, 2015 at 05:18:30PM +0200, Peter Zijlstra wrote:
> Hi,
> 
> IIRC Paul relies on schedule() implying a full memory barrier with
> strong transitivity for RCU.
> 
> If not, ignore this email.

Not so sure about schedule(), but definitely need strong transitivity
for the rcu_node structure's ->lock field.  And the atomic operations
on the rcu_dyntick structure's fields when entering or leaving the
idle loop.

With schedule, the thread later reports the quiescent state, which
involves acquiring the rcu_node structure's ->lock field.  So I -think-
that the locks in the scheduler can be weakly transitive.

> If so, however, I suspect AARGH64 is borken and would need (just like
> PPC):
> 
> #define smp_mb__before_spinlock()	smp_mb()
> 
> The problem is that schedule() (when a NO-OP) does:
> 
> 	smp_mb__before_spinlock();
> 	LOCK rq->lock
> 
> 	clear_bit()
> 
> 	UNLOCK rq->lock
> 
> And nothing there implies a full barrier on AARGH64, since
> smp_mb__before_spinlock() defaults to WMB, LOCK is an "ldaxr" or
> load-acquire, UNLOCK is "stlrh" or store-release and clear_bit() isn't
> anything.
> 
> Pretty much every other arch has LOCK implying a full barrier, either
> because its strongly ordered or because it needs one for the ACQUIRE
> semantics.

Well, arm64 might well need smp_mb__after_unlock_lock() to be non-empty.
But I thought that it used a dmb in the spinlock code somewhere or
another...

							Thanx, Paul

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