On Tue, Feb 16, 2016 at 09:53:20AM +0000, Will Deacon wrote:

On Mon, Feb 15, 2016 at 12:35:12PM -0800, Paul E. McKenney wrote:

quoted

On Mon, Feb 15, 2016 at 06:58:32PM +0000, Will Deacon wrote:

quoted

On Mon, Feb 15, 2016 at 09:58:25AM -0800, Paul E. McKenney wrote:

quoted

Some architectures provide local transitivity for a chain of threads doing
writes separated by smp_wmb(), as exemplified by the litmus tests below.
The pattern is that each thread writes to a its own variable, does an
smp_wmb(), then writes a different value to the next thread's variable.

I don't know of a use of this, but if everyone supports it, it might
be good to mandate it.  Status quo is that smp_wmb() is non-transitive,
so it currently isn't supported.

Anyone know of any architectures that do -not- support this?

Assuming all architectures -do- support this, any arguments -against-
officially supporting it in Linux?

							Thanx, Paul

------------------------------------------------------------------------

Two threads:

	int a, b;

	void thread0(void)
	{
		WRITE_ONCE(a, 1);
		smp_wmb();
		WRITE_ONCE(b, 2);
	}

	void thread1(void)
	{
		WRITE_ONCE(b, 1);
		smp_wmb();
		WRITE_ONCE(a, 2);
	}

	/* After all threads have completed and the dust has settled... */

	BUG_ON(a == 1 && b == 1);

My understanding is that this test, and the generalisation to n threads,
is forbidden on ARM. However, the transitivity of DMB ST (used to
construct smp_wmb()) has been the subject of long debates, because we
allow the following test:


P0:
Wx = 1

P1:
Rx == 1
DMB ST
Wy = 1

P2:
Ry == 1
<addr dep>
Rx == 0


so I'd be uneasy about saying "it's all transitive".

Agreed!  For one thing, doesn't DMB ST need writes on both sides?

Yes, but it's a common trap that people fall into where they think the
above is forbidden because the DMB ST in P1 should order P0's write
before its own write of y.

True enough.

quoted

But that is one reason that I am only semi-enthusiastic about this.
The potentially locally transitive case is -very- restrictive, applying
only to situations where -all- accesses are writes.

I think that we will confuse people more by trying to describe the
restricted case where we provide order than if we blanket say that its
not transitive. I know Linus prefers to be as strong as possible, but
this doesn't look like a realistic programming paradigm and having a
straightforward rule that "rmb and wmb are not transitive" is much
easier for people to deal with in my opinion.

That is a good explanation of why I am only semi-enthusiastic about
this.  ;-)

							Thanx, Paul