Re: Current LKMM patch disposition

["Paul E. McKenney" <paulmck@kernel.org>]

On Sat, Feb 18, 2023 at 01:13:59AM -0500, Joel Fernandes wrote:
> Hi Alan,
> 
> On Sat, Feb 11, 2023 at 9:59 PM Alan Stern <stern@rowland.harvard.edu> wrote:
> >
> [...]
> >
> > Would you like to post a few examples showing some of the most difficult
> > points you encountered?  Maybe explanation.txt can be improved.
> 
> One additional feedback I wanted to mention, regarding this paragraph
> under "WARNING":
> ===========
> The protections provided by READ_ONCE(), WRITE_ONCE(), and others are
> not perfect; and under some circumstances it is possible for the
> compiler to undermine the memory model. Here is an example. Suppose
> both branches of an "if" statement store the same value to the same
> location:
> r1 = READ_ONCE(x);
> if (r1) {
> WRITE_ONCE(y, 2);
> ... /* do something */
> } else {
> WRITE_ONCE(y, 2);
> ... /* do something else */
> }
> ===========
> 
> I tried lots of different compilers with varying degrees of
> optimization, in all cases I find that the conditional instruction
> always appears in program order before the stores inside the body of
> the conditional. So I am not sure if this is really a valid concern on
> current compilers, if not - could you provide an example of a compiler
> and options that cause it?
> 
> In any case, if it is a theoretical concern, it could be clarified
> that this is a theoretical possibility in the text.  And if it is a
> real/practical concern, then it could be mentioned the specific
> compiler/arch this was seen in.

I could be misremembering, but I believe that this reordering has been
seen in the past.

							Thanx, Paul

> Thanks!
> 
>  - Joel
> 
> 
> 
> >
> > > > I'm not sure that breaking this relation up into pieces will make it any
> > > > easier to understand.
> > >
> > > Yes, but I tried. I will keep trying to understand your last patch
> > > more. Especially I am still not sure, why in the case of an SRCU
> > > reader on a single CPU, the following does not work:
> > > let srcu-rscs = ([Srcu-lock]; data; [Srcu-unlock]).
> >
> > You have to understand that herd7 does not track dependencies through
> > stores and subsequent loads.  That is, if you have something like:
> >
> >         r1 = READ_ONCE(*x);
> >         WRITE_ONCE(*y, r1);
> >         r2 = READ_ONCE(*y);
> >         WRITE_ONCE(*z, r2);
> >
> > then herd7 will realize that the write to y depends on the value read
> > from x, and it will realize that the write to z depends on the value
> > read from y.  But it will not realize that the write to z depends on the
> > value read from x; it loses track of that dependency because of the
> > intervening store/load from y.
> >
> > More to the point, if you have:
> >
> >         r1 = srcu_read_lock(lock);
> >         WRITE_ONCE(*y, r1);
> >         r2 = READ_ONCE(*y);
> >         srcu_read_unlock(lock, r2);
> >
> > then herd7 will not realize that the value of r2 depends on the value of
> > r1.  So there will be no data dependency from the srcu_read_lock() to
> > the srcu_read_unlock().
> >
> > Alan