Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

[Alan Stern]

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