Messages in this thread | | | From | Joel Fernandes <> | Date | Fri, 16 Aug 2019 16:44:10 -0400 | Subject | Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates |
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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.
Perhaps the commit message can be reworded to mention that the _ONCE is an additional clean up for safe access.
thanks,
- Joel
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