Re: Control dependency between prior load in while condition and later store?

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 04, 2018 at 04:35:32PM -0400, Alan Stern wrote:
> On Wed, 4 Apr 2018, Daniel Jordan wrote:
> 
> > A question for memory-barriers.txt aficionados.
> > 
> > Is there a control dependency between the prior load of 'a' and the 
> > later store of 'c'?:
> > 
> >    while (READ_ONCE(a));
> >    WRITE_ONCE(c, 1);
> 
> I would say that yes, there is.

Indeed.

> Yes, except that a more accurate view of the object code would be
> something like this:
> 
> Loop:	r1 = READ_ONCE(a);
> 	if (r1)
> 		goto Loop;
> 	else
> 		;	// Do nothing
> 	WRITE_ONCE(c, 1);
> 
> Here you can see that one path branches backward, so everything 
> following the "if" is dependent on the READ_ONCE.

Agreed, and I think I even have code that relies on such a pattern
somewhere.. Ah.. yes, see smp_cond_load_acquire().