Messages in this thread | | | Date | Fri, 16 Aug 2019 16:57:40 -0400 | From | Joel Fernandes <> | Subject | Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates |
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On Fri, Aug 16, 2019 at 10:49:04PM +0200, Thomas Gleixner wrote: > On Fri, 16 Aug 2019, Joel Fernandes wrote: > > On Fri, Aug 16, 2019 at 3:19 PM Alan Stern <stern@rowland.harvard.edu> wrote: > > > On Fri, 16 Aug 2019, Mathieu Desnoyers wrote: > > > > > > > If you choose not to use READ_ONCE(), then the "load tearing" issue can > > > > cause similar spurious 1 -> 0 -> 1 transitions near 16-bit counter > > > > overflow as described above. The "Invented load" also becomes an issue, > > > > because the compiler could use the loaded value for a branch, and re-load > > > > that value between two branches which are expected to use the same value, > > > > effectively generating a corrupted state. > > > > > > > > I think we need a statement about whether READ_ONCE/WRITE_ONCE should > > > > be used in this kind of situation, or if we are fine dealing with the > > > > awkward compiler side-effects when they will occur. > > > > > > The only real downside (apart from readability) of READ_ONCE and > > > WRITE_ONCE is that they prevent the compiler from optimizing accesses > > > to the location being read or written. But if you're just doing a > > > single access in each place, not multiple accesses, then there's > > > nothing to optimize anyway. So there's no real reason not to use > > > READ_ONCE or WRITE_ONCE. > > > > I am also more on the side of using *_ONCE. To me, by principal, I > > would be willing to convert any concurrent plain access using _ONCE, > > just so we don't have to worry about it now or in the future and also > > documents the access. > > By that argumentation we need to plaster half of the kernel with _ONCE() > and I'm so not looking forward to the insane amount of script kiddies > patches to do that.
Really? That is quite scary that you are saying half of the kernel has issues with concurrent access or compiler optimizations. It scares me that a concurrent access can tear down a store/load and existing code can just fail, if that is the case.
> Can we finally put a foot down and tell compiler and standard committee > people to stop this insanity?
Sure, or could the compilers provide flags which prevent such optimization similar to -O* flags?
thanks,
- Joel
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