Re: [PATCH 0/24] make atomic_read() behave consistently across allarchitectures

[Paul Mackerras]

Satyam Sharma writes:

> > Doesn't "atomic WRT all processors" require volatility?
> 
> No, it definitely doesn't. Why should it?
> 
> "Atomic w.r.t. all processors" is just your normal, simple "atomicity"
> for SMP systems (ensure that that object is modified / set / replaced
> in main memory atomically) and has nothing to do with "volatile"
> behaviour.

Atomic variables are "volatile" in the sense that they are liable to
be changed at any time by mechanisms that are outside the knowledge of
the C compiler, namely, other CPUs, or this CPU executing an interrupt
routine.

In the kernel we use atomic variables in precisely those situations
where a variable is potentially accessed concurrently by multiple
CPUs, and where each CPU needs to see updates done by other CPUs in a
timely fashion.  That is what they are for.  Therefore the compiler
must not cache values of atomic variables in registers; each
atomic_read must result in a load and each atomic_set must result in a
store.  Anything else will just lead to subtle bugs.

I have no strong opinion about whether or not the best way to achieve
this is through the use of the "volatile" C keyword.  Segher's idea of
using asm instead seems like a good one to me.

Paul.
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