On Thu, Oct 24, 2019 at 4:40 PM Andrea Parri firstname.lastname@example.org wrote:
On Thu, Oct 24, 2019 at 03:58:40PM +0200, Dmitry Vyukov wrote:
On Thu, Oct 24, 2019 at 3:43 PM Andrea Parri email@example.com wrote:
But why? I think kernel contains lots of such cases and it seems to be officially documented by the LKMM: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tool... address dependencies and ppo
Well, that same documentation also alerts about some of the pitfalls developers can incur while relying on dependencies. I'm sure you're more than aware of some of the debate surrounding these issues.
I thought that LKMM is finally supposed to stop all these centi-threads around subtle details of ordering. And not we finally have it. And it says that using address-dependencies is legal. And you are one of the authors. And now you are arguing here that we better not use it :) Can we have some black/white yes/no for code correctness reflected in LKMM please :) If we are banning address dependencies, don't we need to fix all of rcu uses?
Current limitations of the LKMM are listed in tools/memory-model/README (and I myself discussed a number of them at LPC recently); the relevant point here seems to be:
Compiler optimizations are not accurately modeled. Of course, the use of READ_ONCE() and WRITE_ONCE() limits the compiler's ability to optimize, but under some circumstances it is possible for the compiler to undermine the memory model. [...] Note that this limitation in turn limits LKMM's ability to accurately model address, control, and data dependencies.
A less elegant, but hopefully more effective, way to phrase such point is maybe "feel free to rely on dependencies, but then do not blame the LKMM authors please". ;-)
We are not going to blame LKMM authors :)
Acquire will introduce actual hardware barrier on arm/power/etc. Maybe it does not matter here. But I feel if we start replacing all load-depends/rcu with acquire, it will be noticeable overhead. So what do we do in the context of the whole kernel?