Messages in this thread | | | Date | Thu, 16 Aug 2007 09:22:44 +1000 | From | Paul Mackerras <> | Subject | Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures |
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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.
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