Re: [v3,11/41] mips: reuse asm-generic/barrier.h

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jan 26, 2016 at 11:44:46AM -0800, Linus Torvalds wrote:
> On Tue, Jan 26, 2016 at 9:22 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > This is distinct from:
> 
> That may be distinct, but:
> 
> >         struct foo *x = READ_ONCE(*ptr);
> >         smp_read_barrier_depends();
> >         x->bar = 5;
> 
> This case is complete BS. Stop perpetuating it. I already removed a
> number of bogus cases of it, and I removed the incorrect documentation
> that had this crap.

If I understand your objection correctly, you want the above pattern
expressed either like this:

	struct foo *x = rcu_dereference(*ptr);
	x->bar = 5;

Or like this:

	struct foo *x = lockless_dereference(*ptr);
	x->bar = 5;

Or am I missing your point?

> It's called "smp_READ_barrier_depends()" for a reason.
> 
> Alpha is the only one that needs it, and alpha needs it only for
> dependent READS.
> 
> It's not called smp_read_write_barrier_depends(). It's not called
> "smp_mb_depends()". It's a weaker form of "smp_rmb()", nothing else.
> 
> So alpha does have an implied dependency chain from a read to a
> subsequent dependent write, and does not need any extra barriers.
> 
> Alpha does *not* have a dependency chain from a read to a subsequent
> read, which is why we need that horrible crappy
> smp_read_barrier_depends(). But it's the only reason.
> 
> This is the alpha reference manual wrt read-to-write dependency:
> 
>   5.6.1.7 Definition of Dependence Constraint
> 
>     The depends relation (DP) is defined as follows. Given u and v
> issued by processor Pi, where u
>     is a read or an instruction fetch and v is a write, u precedes v
> in DP order (written u DP v, that
>     is, v depends on u) in either of the following situations:
> 
>      • u determines the execution of v, the location accessed by v, or
> the value written by v.
>      • u determines the execution or address or value of another
> memory access z that precedes
> 
>     v or might precede v (that is, would precede v in some execution
> path depending
>     on the value read by u) by processor issue constraint (see Section 5.6.1.3).
> 
> Note that the dependence barrier honors not only control flow, but
> address and data values too.  This is a different syntax than we use,
> but 'u' is the READ_ONCE, and 'v' is the write. Any data, address or
> conditional dependency between the two implies an ordering.
> 
> So no, "smp_read_barrier_depends()" is *ONLY* about two reads, where
> the second read is data-dependent on the first. Nothing else.
> 
> So if you _ever_ see a "smp_read_barrier_depends()" that isn't about a
> barrier between two reads, then that is a bug.

And the smp_read_barrier_depends() in both rcu_dereference() and
in lockless_dereference() is ordering the read-to-read case and the
underlying hardware is ordering the read-to-write case on weakly ordered
hardware.

Or, again, am I missing your point?

							Thanx, Paul

> The above code is crap.  It's exactly as much crap as
> 
>    a = READ_ONCE(x);
>    smp_rmb();
>    WRITE_ONCE(b, y);
> 
> because a "rmb()" simply doesn't have anything to do with
> read-vs-subsequent-write ordering.
> 
>                  Linus
>