From: Paul Heidekrüger <paul.heidekrueger@xxxxxxxxx> As discussed, clarify LKMM not recognizing certain kinds of orderings. In particular, highlight the fact that LKMM might deliberately make weaker guarantees than compilers and architectures. [ paulmck: Fix whitespace issue noted by checkpatch.pl. ] Link: https://lore.kernel.org/all/YpoW1deb%2FQeeszO1@xxxxxxxxxxxxxxxxxxxxxxxx/T/#u Co-developed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@xxxxxxxxx> Reviewed-by: Marco Elver <elver@xxxxxxxxxx> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> Cc: Charalampos Mainas <charalampos.mainas@xxxxxxxxx> Cc: Pramod Bhatotia <pramod.bhatotia@xxxxxxxxx> Cc: Soham Chakraborty <s.s.chakraborty@xxxxxxxxxx> Cc: Martin Fink <martin.fink@xxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> --- .../Documentation/litmus-tests.txt | 37 ++++++++++++++----- 1 file changed, 27 insertions(+), 10 deletions(-) diff --git a/tools/memory-model/Documentation/litmus-tests.txt b/tools/memory-model/Documentation/litmus-tests.txt index 8a9d5d2787f9e..26554b1c5575e 100644 --- a/tools/memory-model/Documentation/litmus-tests.txt +++ b/tools/memory-model/Documentation/litmus-tests.txt @@ -946,22 +946,39 @@ Limitations of the Linux-kernel memory model (LKMM) include: carrying a dependency, then the compiler can break that dependency by substituting a constant of that value. - Conversely, LKMM sometimes doesn't recognize that a particular - optimization is not allowed, and as a result, thinks that a - dependency is not present (because the optimization would break it). - The memory model misses some pretty obvious control dependencies - because of this limitation. A simple example is: + Conversely, LKMM will sometimes overestimate the amount of + reordering compilers and CPUs can carry out, leading it to miss + some pretty obvious cases of ordering. A simple example is: r1 = READ_ONCE(x); if (r1 == 0) smp_mb(); WRITE_ONCE(y, 1); - There is a control dependency from the READ_ONCE to the WRITE_ONCE, - even when r1 is nonzero, but LKMM doesn't realize this and thinks - that the write may execute before the read if r1 != 0. (Yes, that - doesn't make sense if you think about it, but the memory model's - intelligence is limited.) + The WRITE_ONCE() does not depend on the READ_ONCE(), and as a + result, LKMM does not claim ordering. However, even though no + dependency is present, the WRITE_ONCE() will not be executed before + the READ_ONCE(). There are two reasons for this: + + The presence of the smp_mb() in one of the branches + prevents the compiler from moving the WRITE_ONCE() + up before the "if" statement, since the compiler has + to assume that r1 will sometimes be 0 (but see the + comment below); + + CPUs do not execute stores before po-earlier conditional + branches, even in cases where the store occurs after the + two arms of the branch have recombined. + + It is clear that it is not dangerous in the slightest for LKMM to + make weaker guarantees than architectures. In fact, it is + desirable, as it gives compilers room for making optimizations. + For instance, suppose that a 0 value in r1 would trigger undefined + behavior elsewhere. Then a clever compiler might deduce that r1 + can never be 0 in the if condition. As a result, said clever + compiler might deem it safe to optimize away the smp_mb(), + eliminating the branch and any ordering an architecture would + guarantee otherwise. 2. Multiple access sizes for a single variable are not supported, and neither are misaligned or partially overlapping accesses. -- 2.31.1.189.g2e36527f23
