Messages in this thread | | | Date | Thu, 7 Jun 2018 11:32:01 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH V5] powercap/drivers/idle_injection: Add an idle injection framework |
| |
On Thu, Jun 07, 2018 at 11:09:13AM +0200, Daniel Lezcano wrote: > On 07/06/2018 10:49, Viresh Kumar wrote: > > On 07-06-18, 10:46, Daniel Lezcano wrote: > >> Yes, correct. > >> > >> But if we don't care about who wins to store to value, is there a risk > >> of scramble variable if we just assign a value ? > > > > Normally no, as the compiler wouldn't screw it up badly. But there is no rule > > which stops the compiler from doing this: > > > > idle_duration_ms = 5; > > idle_duration_ms = -5; > > idle_duration_ms = 0; > > idle_duration_ms = <real-value-we-want-to-write>; > > > > So we *must* use READ/WRITE_ONCE() to make sure garbage values aren't seen by > > readers. > > Ok understood. Why would a compiler do this kind of things ?
I think the above can happen when the compiler uses the variable as a scratch pad -- very rare I would say.
In general a compiler needs to proof that doing this makes no observable difference ("as-if" rule). And since it is a regular variable it can assume data-race-free and do the above (or something like that). Because if there is a data-race it is UB and it can still do whatever it pleases.
And here I think the point is that regular variables are considered only in the context of a single linear execution context. Locks are assumed to bound observability.
And here the "volatile" and "_atomic" type specifiers again tell the compiler something 'special' is going on and you should not muck with things.
| |