On Sun, Jul 28, 2019 at 10:48:51AM -0400, Alan Stern wrote: > On Sat, 27 Jul 2019, Joel Fernandes (Google) wrote: > > > The lkmm example about ->prop relation should describe an additional rfe > > link between P1's store to y and P2's load of y, which should be > > critical to establishing the ordering resulting in the ->prop ordering > > on P0. IOW, there are 2 rfe links, not one. > > > > Correct these in the docs to make the ->prop ordering on P0 more clear. > > > > Cc: kernel-t...@android.com > > Reviewed-by: Boqun Feng <boqun.f...@gmail.com> > > Signed-off-by: Joel Fernandes (Google) <j...@joelfernandes.org> > > --- > > This is not a good update. See below...
No problem, thanks for the feedback. I am new to the LKMM so please bear with me.. I should have tagged this RFC. > > .../memory-model/Documentation/explanation.txt | 17 ++++++++++------- > > 1 file changed, 10 insertions(+), 7 deletions(-) > > > > diff --git a/tools/memory-model/Documentation/explanation.txt > > b/tools/memory-model/Documentation/explanation.txt > > index 68caa9a976d0..aa84fce854cc 100644 > > --- a/tools/memory-model/Documentation/explanation.txt > > +++ b/tools/memory-model/Documentation/explanation.txt > > @@ -1302,8 +1302,8 @@ followed by an arbitrary number of cumul-fence links, > > ending with an > > rfe link. You can concoct more exotic examples, containing more than > > one fence, although this quickly leads to diminishing returns in terms > > of complexity. For instance, here's an example containing a coe link > > -followed by two fences and an rfe link, utilizing the fact that > > -release fences are A-cumulative: > > +followed by a fence, an rfe link, another fence and and a final rfe link, > ^---^ > > +utilizing the fact that release fences are A-cumulative: > > I don't like this, for two reasons. First is the repeated "and" typo. Will fix the trivial typo, sorry about that. > More importantly, it's not necessary to go into this level of detail; a > better revision would be: > > +followed by two cumul-fences and an rfe link, utilizing the fact that > > This is appropriate because the cumul-fence relation is defined to > contain the rfe link which you noticed wasn't mentioned explicitly. No, I am talking about the P1's store to Y and P2's load of Y. That is not through a cumul-fence so I don't understand what you meant? That _is_ missing the rfe link I am referring to, that is left out. The example says r2 = 1 and then we work backwards from that. r2 could very well have been 0, there's no fence or anything involved, it just happens to be the executation pattern causing r2 = 1 and hence the rfe link. Right? > > int x, y, z; > > > > @@ -1334,11 +1334,14 @@ If x = 2, r0 = 1, and r2 = 1 after this code runs > > then there is a prop > > link from P0's store to its load. This is because P0's store gets > > overwritten by P1's store since x = 2 at the end (a coe link), the > > smp_wmb() ensures that P1's store to x propagates to P2 before the > > -store to y does (the first fence), the store to y propagates to P2 > > -before P2's load and store execute, P2's smp_store_release() > > -guarantees that the stores to x and y both propagate to P0 before the > > -store to z does (the second fence), and P0's load executes after the > > -store to z has propagated to P0 (an rfe link). > > +store to y does (the first fence), P2's store to y happens before P2's > ---------------------------------------^ > > This makes no sense, since P2 doesn't store to y. You meant P1's store > to y. Also, the use of "happens before" is here unnecessarily > ambiguous (is it an informal usage or does it refer to the formal > happens-before relation?). The original "propagates to" is better. Will reword this. > > +load of y (rfe link), P2's smp_store_release() ensures that P2's load > > +of y executes before P2's store to z (second fence), which implies that > > +that stores to x and y propagate to P2 before the smp_store_release(), > > which > > +means that P2's smp_store_release() will propagate stores to x and y to all > > +CPUs before the store to z propagates (A-cumulative property of this > > fence). > > +Finally P0's load of z executes after P2's store to z has propagated to > > +P0 (rfe link). > > Again, a better change would be simply to replace the two instances of > "fence" in the original text with "cumul-fence". Ok that's fine. But I still feel the rfe is not a part of the cumul-fence. The fences have nothing to do with the rfe. Or, I am missing something quite badly. Boqun, did you understand what Alan is saying? thanks, - Joel
