On Sun, Oct 04, 2020 at 12:16:31AM +0900, Akira Yokosawa wrote: > Hi Alan, > > Just a minor nit in the litmus test. > > On Sat, 3 Oct 2020 09:22:12 -0400, Alan Stern wrote: > > To expand on my statement about the LKMM's weakness regarding control > > constructs, here is a litmus test to illustrate the issue. You might > > want to add this to one of the archives. > > > > Alan > > > > C crypto-control-data > > (* > > * LB plus crypto-control-data plus data > > * > > * Expected result: allowed > > * > > * This is an example of OOTA and we would like it to be forbidden. > > * The WRITE_ONCE in P0 is both data-dependent and (at the hardware level) > > * control-dependent on the preceding READ_ONCE. But the dependencies are > > * hidden by the form of the conditional control construct, hence the > > * name "crypto-control-data". The memory model doesn't recognize them. > > *) > > > > {} > > > > P0(int *x, int *y) > > { > > int r1; > > > > r1 = 1; > > if (READ_ONCE(*x) == 0) > > r1 = 0; > > WRITE_ONCE(*y, r1); > > } > > > > P1(int *x, int *y) > > { > > WRITE_ONCE(*x, READ_ONCE(*y)); > > Looks like this one-liner doesn't provide data-dependency of y -> x on herd7. You're right. This is definitely a bug in herd7. Luc, were you aware of this? > When I changed P1 to > > P1(int *x, int *y) > { > int r1; > > r1 = READ_ONCE(*y); > WRITE_ONCE(*x, r1); > } > > and replaced the WRITE_ONCE() in P0 with smp_store_release(), > I got the result of: > > ----- > Test crypto-control-data Allowed > States 1 > 0:r1=0; > No > Witnesses > Positive: 0 Negative: 3 > Condition exists (0:r1=1) > Observation crypto-control-data Never 0 3 > Time crypto-control-data 0.01 > Hash=9b9aebbaf945dad8183d2be0ccb88e11 > ----- > > Restoring the WRITE_ONCE() in P0, I got the result of: > > ----- > Test crypto-control-data Allowed > States 2 > 0:r1=0; > 0:r1=1; > Ok > Witnesses > Positive: 1 Negative: 4 > Condition exists (0:r1=1) > Observation crypto-control-data Sometimes 1 4 > Time crypto-control-data 0.01 > Hash=843eaa4974cec0efae79ce3cb73a1278 > ----- What you should have done was put smp_store_release in P0 and left P1 in its original form. That test should not be allowed, but herd7 says that it is. > As this is the same as the expected result, I suppose you have missed another > limitation of herd7 + LKMM. It would be more accurate to say that we all missed it. :-) (And it's a bug in herd7, not a limitation of either herd7 or LKMM.) How did you notice it? > By the way, I think this weakness on control dependency + data dependency > deserves an entry in tools/memory-model/Documentation/litmus-tests.txt. > > In the LIMITATIONS section, item #1 mentions some situation where > LKMM may not recognize possible losses of control-dependencies by > compiler optimizations. > > What this litmus test demonstrates is a different class of mismatch. Yes, one in which LKMM does not recognize a genuine dependency because it can't tell that some optimizations are not valid. This flaw is fundamental to the way herd7 works. It examines only one execution at a time, and it doesn't consider the code in a conditional branch while it's examining an execution where that branch wasn't taken. Therefore it has no way to know that the code in the unexecuted branch would prevent a certain optimization. But the compiler does consider all the code in all branches when deciding what optimizations to apply. Here's another trivial example: r1 = READ_ONCE(*x); if (r1 == 0) smp_mb(); WRITE_ONCE(*y, 1); The compiler can't move the WRITE_ONCE before the READ_ONCE or the "if" statement, because it's not allowed to move shared memory accesses past a memory barrier -- even if that memory barrier isn't always executed. Therefore the WRITE_ONCE actually is ordered after the READ_ONCE, but the memory model doesn't realize it. > Alan, can you come up with an update in this regard? I'll write something. Alan