Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

[Satyam Sharma <satyam@infradead.org>]

On Fri, 17 Aug 2007, Nick Piggin wrote:

> Satyam Sharma wrote:
> [...]
> > You think both these are equivalent in terms of "looks":
> > 
> > 					|
> > while (!atomic_read(&v)) {		|	while (!atomic_read_xxx(&v)) {
> > 	...				|		...
> > 	cpu_relax_no_barrier();		|
> > cpu_relax_no_barrier();
> > 	order_atomic(&v);		|	}
> > }					|
> > 
> > (where order_atomic() is an atomic_t
> > specific wrapper as you mentioned below)
> > 
> > ?
> 
> I think the LHS is better if your atomic_read_xxx primitive is using the
> crazy one-sided barrier,
  ^^^^^

I'd say it's purposefully one-sided.

> because the LHS code you immediately know what
> barriers are happening, and with the RHS you have to look at the
> atomic_read_xxx
> definition.

No. As I said, the _xxx (whatever the heck you want to name it as) should
give the same heads-up that your "order_atomic" thing is supposed to give.


> If your atomic_read_xxx implementation was more intuitive, then both are
> pretty well equal. More lines != ugly code.
> 
> > [...]
> > What bugs?
> 
> You can't think for yourself? Your atomic_read_volatile contains a compiler
> barrier to the atomic variable before the load. 2 such reads from different
> locations look like this:
> 
> asm volatile("" : "+m" (v1));
> atomic_read(&v1);
> asm volatile("" : "+m" (v2));
> atomic_read(&v2);
> 
> Which implies that the load of v1 can be reordered to occur after the load
> of v2.

And how would that be a bug? (sorry, I really can't think for myself)


> > > Secondly, what sort of code would do such a thing?
> > 
> > See the nodemgr_host_thread() that does something similar, though not
> > exactly same.
> 
> I'm sorry, all this waffling about made up code which might do this and
> that is just a waste of time.

First, you could try looking at the code.

And by the way, as I've already said (why do *require* people to have to
repeat things to you?) this isn't even about only existing code.
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