Messages in this thread | | | Subject | Re: [PATCH V5] powercap/drivers/idle_injection: Add an idle injection framework | From | Daniel Lezcano <> | Date | Thu, 7 Jun 2018 14:31:54 +0200 |
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On 07/06/2018 11:39, Peter Zijlstra wrote: > On Thu, Jun 07, 2018 at 11:32:01AM +0200, Peter Zijlstra wrote: >> 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. > > Also, I think, more likely: > > if (cond) > X = 5; > else > X = 4; > > is allowed to be transformed into: > > X = 4; > if (cond) > X = 5; > > as long as cond doesn't involve a sequence point of sorts (think > function call). > > For the single execution context case, this transformation is valid, but > it is not in the threaded case. But then we go back to the assumption > that regular variables are data-race-free.
Thank you very much for the explanations.
-- Daniel
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