Writes, smp_wmb(), and transitivity?

[Paul E. McKenney]

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:
> > Hello!
> 
> Hi Paul,
> 
> > 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