[PATCH v2] barriers: introduce smp_mb__release_acquire and update documentation

[David Laight]

From: Paul E. McKenney
> Sent: 21 October 2015 00:35
...
> There is also the question of whether the barrier forces ordering
> of unrelated stores, everything initially zero and all accesses
> READ_ONCE() or WRITE_ONCE():
> 
> 	P0		P1		P2		P3
> 	X = 1;		Y = 1;		r1 = X;		r3 = Y;
> 					some_barrier();	some_barrier();
> 					r2 = Y;		r4 = X;
> 
> P2's and P3's ordering could be globally visible without requiring
> P0's and P1's independent stores to be ordered, for example, if you
> used smp_rmb() for some_barrier().  In contrast, if we used smp_mb()
> for barrier, everyone would agree on the order of P0's and P0's stores.
> 
> There are actually a fair number of different combinations of
> aspects of memory ordering.  We will need to choose wisely.  ;-)

My thoughts on this are that most code probably isn't performance critical
enough to be using anything other than normal locks for inter-cpu
synchronisation.
Certainly most people are likely to get it wrong somewhere.
So you want a big red sticker saying 'Don't try to be too clever'.

Also without examples of why things go wrong (eg member_consumer()
and alpha) it is difficult to understand the differences between
all the barriers (etc).

OTOH device driver code may need things slightly stronger than
barrier() (which I think is asm(:::"memory")) to sequence accesses
to hardware devices (and memory the hardware reads), but without
having a strong barrier in every ioread/write() access.

	David