On Tue, Nov 19, 2019 at 11:45:03PM +0800, laokz wrote:
> Hello paul,
>
> 在 2019-11-19二的 06:44 -0800，Paul E. McKenney写道：
> > On Tue, Nov 19, 2019 at 10:11:48PM +0800, laokz wrote:
> > > But wonder how about cpu in-order commit. So I removed smp_mb(), the
> > > litmus
> > > test showed the result exist! This confused me. In my understanding, cpu
> > > in-
> > > order commit means out-of-order-execution results commit to
> > > register/memory
> > > in compiled program order. That is cpu P1 must retire r0=y first then
> > > x=1,
> > > thus P0 can see P1's update of x. So P1's r0 should never be 1.
> > >
> > > Is this caused by LKMM's compatibility with out-of-order commit
> > > architectures? Or what's wrong with me?
> >
> > Nothing is wrong with you. You are just going through a common phase
> > in learning about memory models. ;-)
> >
> > So you modified P1() as follows, correct?
> >
> > P1(int *x, int *y)
> > {
> > int r0;
> >
> > r0 = READ_ONCE(*y);
> > WRITE_ONCE(*x, 1);
> > }
> >
> > The compiler is free to rearrange this code as follows:
> >
> > P1(int *x, int *y)
> > {
> > int r0;
> >
> > WRITE_ONCE(*x, 1);
> > r0 = READ_ONCE(*y);
> > }
> >
> > This can clearly satisfy the exists clause: P1() does its write,
> > P0() does its read and its write, and finally P1() does its read.
>
> Compiler is one big deal. It seems that seldom compilers obey C11
> standard strictly. "Accesses to volatile objects are evaluated strictly
> according to the rules of the abstract machine." at least means they should
> not change the sequence point.
Yes, you are right, the volatile nature of READ_ONCE() and WRITE_ONCE()
would prohibit the reordering above. On the other hand, the C++11
standard really does allow relaxed atomic loads and stores to be
reordered. And since I was at a C++ standards committee a few weeks
ago, I had relaxed atomics on my brain. Apologies for my confusion.
> > But suppose we prevented the compiler from moving the code:
> >
> > P1(int *x, int *y)
> > {
> > int r0;
> >
> > r0 = READ_ONCE(*y);
> > barrier();
> > WRITE_ONCE(*x, 1);
> > }
> >
> > Then, as you say, weakly ordered CPUs might still reorder P1()'s
> > read and write. So LKMM must still say that the exists clause is
> > satisfied.
>
> Legacy resource is another big deal.
I will let you argue the "Legacy resource" point with the vendors still
selling weakly ordered CPUs. ;-)
> Thanks your quick reply. It really clears my head.
;-) ;-) ;-)
Thanx, Paul
