Messages in this thread | | | Date | Fri, 16 Aug 2019 15:19:43 -0400 (EDT) | From | Alan Stern <> | Subject | Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates |
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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.
Alan Stern
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