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

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

quoted

Hello!

Hi Paul,

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?

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.

							Thanx, Paul