Re: [PATCH tip/core/rcu 3/4] documentation: Add acquire/release barriers to pairing rules

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

On Wed, Jul 16, 2014 at 03:05:34PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 16, 2014 at 05:16:26AM -0700, Paul E. McKenney wrote:
> > On Mon, Jul 14, 2014 at 01:57:38PM +0200, Peter Zijlstra wrote:
> > > On Tue, Jul 08, 2014 at 08:31:17AM -0700, Paul E. McKenney wrote:
> > > 
> > > > Good point, how about the following?
> > > > 
> > > > 	General barriers pair with each other, though they also pair
> > > > 	with most other types of barriers, albeit without transitivity.
> > > 
> > > > 	An acquire barrier pairs with a release barrier, but both may also
> > > > 	pair with other barriers, including of course general barriers.
> > > 
> > > > 	A write barrier pairs with a data dependency barrier, an acquire
> > > > 	barrier, a release barrier, a read barrier, or a general barrier.
> > > 
> > > > 	Similarly a read barrier or a data dependency barrier pairs
> > > > 	with a write barrier, an acquire barrier, a release barrier,
> > > > 	or a general barrier:
> > > 
> > > It might be clearer with the added whitespace, or as an explicit list  I
> > > suppose, but yes.
> > 
> > If I get ambitious, I might try making a table out of it, but I am not
> > yet sure how I would set that up.  Something about having to say a lot
> > in each cell, but with only a small amount of room in which to say it.
> 
> 
>       |  mb | wmb | rmb | rbd | acq | rel |
>  -----+-----+-----+-----+-----+-----+-----+
>    mb |  X  |  X  |  X  |  X  |  X  |  X  |
>  -----+-----+-----+-----+-----+-----+-----+
>   wmb |  X  |     |  X  |  X  |     |     |
>  -----+-----+-----+-----+-----+-----+-----+
>   rmb |  X  |  X  |     |     |     |     |
>  -----+-----+-----+-----+-----+-----+-----+
>   rbd |  X  |  X  |     |     |     |     |
>  -----+-----+-----+-----+-----+-----+-----+
>   acq |  X  |     |     |     |     |  X  |
>  -----+-----+-----+-----+-----+-----+-----+
>   rel |  X  |     |     |     |  X  |     |
>  -----+-----+-----+-----+-----+-----+-----+
> 
> (where rbd is read_barrier_depends).
> 
> Which is not entirely filled out, in particular I didn't do the creative
> acq/rel bits.

Also needs to reflect that wmb really can pair with wmb.  See for example
Scenario 15 of https://lwn.net/Articles/573436/.  Then again, your point
might be that Scenario 15 is not all that useful, which is true in my
experience -- though a good way to cause someone to find a use is to leave
it out of such a table...

> > > Ah, I was more thinking of the fact that ACQUIRE/RELEASE are
> > > semi-permeable while READ/WRITE are memop dependent.
> > > 
> > > So any combination will be a semi-permeable memop dependent thing,
> > > which is the most narrow barrier possible.
> > > 
> > > So if we thing of ACQUIRE/RELEASE as being 'half' a full barrier,
> > > separated in direction, and READ/WRITE as being 'half' a full barrier
> > > separated on type, then the combination is a 'quarter' barrier.
> > > 
> > > Not arguing they're not useful, just saying we need to be extra careful.
> > 
> > I do agree completely about the need for extra care!
> > 
> > For whatever it is worth, the permeability and read-write properties
> > are isolated to each barrier in the pair.  For example, with "a" and
> > "b" both initially zero:
> > 
> > 	CPU 1				CPU 2
> > 	-----				-----
> > 	ACCESS_ONCE(a) = 1;		r1 = b;
> > 	smp_store_release(&b, 1);	smp_rmb();
> > 	ACCESS_ONCE(c) = 1;		r2 = a;
> > 					ACCESS_ONCE(c) = 2;
> > 
> > The outcome r1==1&&r2==0 is prohibited, but the ordering of the stores
> > to "c" are not ordered: CPU 1's smp_store_release() does not affect
> > later accesses, and CPU 2's smp_rmb() does not order stores.
> > 
> > Not sure that it is worth adding this sort of example, though.
> 
> Yeah, not sure either. Maybe just a big fat caution if you pair acq/rel
> with anything other than its opposite or a general barrier.
> 
> Maybe use small 'x' for acq/rel + rmb/wmb and put a caution in the
> 'legend' for 'x'.

When I expand things out, I end up wanting each cell to hold a
scenario from the tables in https://lwn.net/Articles/573436/ and
https://lwn.net/Articles/573497/, probably along with some cautions
on what it does not do as well.  :-(

							Thanx, Paul