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SubjectRe: [PATCH v2] make buffer_locked provide an acquire semantics
On Tue, Aug 02, 2022 at 09:54:55AM +0100, Will Deacon wrote:
> On Mon, Aug 01, 2022 at 12:20:35PM -0700, Paul E. McKenney wrote:
> > On Mon, Aug 01, 2022 at 04:41:09PM +0100, Will Deacon wrote:
> > > Apologies for the slow response here; believe it or not, I was attending
> > > a workshop about memory ordering.
> >
> > Nice!!! Anything that I can/should know from that gathering? ;-)
>
> Oh come off it, you know this stuff already ;)

Thank you for the kind words, but the most devastating learning disability
of all is thinking that you already know everything about the topic
in question. ;-)

> > > On Sun, Jul 31, 2022 at 10:30:11AM -0700, Paul E. McKenney wrote:
> > > > On Sun, Jul 31, 2022 at 09:51:47AM -0700, Linus Torvalds wrote:
> > > > > Even alpha is specified to be locally ordered wrt *one* memory
> > > > > location, including for reads (See table 5-1: "Processor issue order",
> > > > > and also 5.6.2.2: "Litmus test 2"). So if a previous read has seen a
> > > > > new value, a subsequent read is not allowed to see an older one - even
> > > > > without a memory barrier.
> > > > >
> > > > > Will, Paul? Maybe that's only for overlapping loads/stores, not for
> > > > > loads/loads. Because maybe alpha for once isn't the weakest possible
> > > > > ordering.
> > > >
> > > > The "bad boy" in this case is Itanium, which can do some VLIW reordering
> > > > of accesses. Or could, I am not sure that newer Itanium hardware
> > > > does this. But this is why Itanium compilers made volatile loads use
> > > > the ld,acq instruction.
> > > >
> > > > Which means that aligned same-sized marked accesses to a single location
> > > > really do execute consistently with some global ordering, even on Itanium.
> > >
> > > Although this is true, there's a really subtle issue which crops up if you
> > > try to compose this read-after-read ordering with dependencies in the case
> > > where the two reads read the same value (which is encapsulated by the
> > > unusual RSW litmus test that I've tried to convert to C below):
> >
> > RSW from the infamous test6.pdf, correct?
>
> That's the badger. I've no doubt that you're aware of it already, but I
> thought it was a useful exercise to transcribe it to C and have it on the
> mailing list for folks to look at.

I have seen it, but this was nevertheless a useful reminder.

> > > /* Global definitions; assume everything zero-initialised */
> > > struct foo {
> > > int *x;
> > > };
> > >
> > > int x;
> > > struct foo foo;
> > > struct foo *ptr;
> > >
> > >
> > > /* CPU 0 */
> > > WRITE_ONCE(x, 1);
> >
> > Your x is RSW's z?
>
> Yes.
>
> > > WRITE_ONCE(foo.x, &x);
> >
> > And your foo.x is RSW's x? If so, the above WRITE_ONCE() could happen at
> > compile time, correct? Or in the initialization clause of a litmus test?
>
> Yes, although I think it's a tiny bit more like real code to have it done
> here, although it means that the "surprising" outcome relies on this being
> reordered before the store to x.
>
> > > /*
> > > * Release ordering to ensure that somebody following a non-NULL ptr will
> > > * see a fully-initialised 'foo'. smp_[w]mb() would work as well.
> > > */
> > > smp_store_release(&ptr, &foo);
> >
> > Your ptr is RSW's y, correct?
>
> Yes.
>
> > > /* CPU 1 */
> > > int *xp1, *xp2, val;
> > > struct foo *foop;
> > >
> > > /* Load the global pointer and check that it's not NULL. */
> > > foop = READ_ONCE(ptr);
> > > if (!foop)
> > > return;
> >
> > A litmus tests can do this via the filter clause.
>
> Indeed, but I was trying to make this look like C code for non-litmus
> speakers!
>
> > > /*
> > > * Load 'foo.x' via the pointer we just loaded. This is ordered after the
> > > * previous READ_ONCE() because of the address dependency.
> > > */
> > > xp1 = READ_ONCE(foop->x);
> > >
> > > /*
> > > * Load 'foo.x' directly via the global 'foo'.
> > > * _This is loading the same address as the previous READ_ONCE() and
> > > * therefore cannot return a stale (NULL) value!_
> > > */
> > > xp2 = READ_ONCE(foo.x);
> >
> > OK, same location, but RSW calls only for po, not addr from the initial
> > read to this read, got it. (My first attempt left out this nuance,
> > in case you were wondering.)
>
> Right, there is only po from the initial read to this read. If there was an
> address dependency, then we'd have a chain of address dependencies from the
> first read to the last read on this CPU and the result (of x == 0) would be
> forbidden.
>
> > > /*
> > > * Load 'x' via the pointer we just loaded.
> > > * _We may see zero here!_
> > > */
> > > val = READ_ONCE(*xp2);
> >
> > And herd7/LKMM agree with this, at least assuming I got the litmus
> > test right. (I used RSW's variables as a cross-check.)
>
> That's promising, but see below...
>
> > C rsw
> >
> > {
> > a=0;
> > x=z;
> > y=a;
> > z=0;
> > }
> >
> > P0(int *x, int **y, int *z)
> > {
> > WRITE_ONCE(*z, 1);
> > WRITE_ONCE(*y, x);
> > }
>
> Ah wait, you need a barrier between these two writes, don't you? I used
> an smp_store_release() but smp[w]_mb() should do too.

You are quite right, thank you! Here is the fixed version and output,
which LKMM still says is allowed.

Thanx, Paul

------------------------------------------------------------------------

C rsw

{
a=0;
x=z;
y=a;
z=0;
}

P0(int *x, int **y, int *z)
{
WRITE_ONCE(*z, 1);
smp_store_release(y, x);
}

P1(int *x, int **y, int *z)
{
r1 = READ_ONCE(*y);
r2 = READ_ONCE(*r1);
r3 = READ_ONCE(*x);
r4 = READ_ONCE(*r3);
}

filter(1:r1=x)
exists(1:r2=z /\ 1:r3=z /\ 1:r4=0)

------------------------------------------------------------------------

$ herd7 -conf linux-kernel.cfg /tmp/rsw.litmus
Test rsw Allowed
States 2
1:r2=z; 1:r3=z; 1:r4=0;
1:r2=z; 1:r3=z; 1:r4=1;
Ok
Witnesses
Positive: 1 Negative: 1
Condition exists (1:r2=z /\ 1:r3=z /\ 1:r4=0)
Observation rsw Sometimes 1 1
Time rsw 0.01
Hash=588486c0f4d521fa3ce559a19ed118d5

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