Messages in this thread | | | Date | Thu, 7 Jun 2018 11:09:28 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH V5] powercap/drivers/idle_injection: Add an idle injection framework |
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On Thu, Jun 07, 2018 at 02:19:21PM +0530, 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.
That too, however it is far worse..
The compiler is allowed to do store/load-tearing. Basically it can emit individual byte store/loads in any random order.
So: foo = bar = 0;
P0 P1
foo = 0x12345678; bar = foo;
Could result in: bar == 0x12005600 or any other random combination.
Now, it generally doesn't do this, because it is really retarded to generate code like that. But we've seen cases where it managed to do really weird things (think constructing 64bit literals with two or more smaller stores, which total smaller code).
The volatile in READ/WRITE_ONCE() disallows this and ensures the variables are read / written in a single go (assuming naturally aligned native word sizes).
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