Re: why are some atomic_t's not volatile, while most are?

[Chris Snook <csnook@redhat.com>]

Chris Friesen wrote:
> Chris Snook wrote:
>
> > But if you're not using SMP, the only way you get a race condition is 
> > if your compiler is reordering instructions that have side effects 
> > which are invisible to the compiler.  This can happen with MMIO 
> > registers, but it's not an issue with an atomic_t we're declaring in 
> > real memory.
>
> I refer back to the interrupt handler case.  Suppose we have:
>
> while(!atomic_read(flag))
>      continue;
>
> where flag is an atomic_t that is set in an interrupt handler, the 
> volatile may be necessary on some architectures to force the compiler to 
> re-read "flag" each time through the loop.
>
> Without the "volatile", the compiler could be perfectly within its 
> rights to evaluate "flag" once and create an infinite loop.
>
> Now I'm not trying to say that we should explictly use "volatile" in 
> common code, but that it is possible that it is required within the 
> arch-specific atomic_t accessors even on uniprocessor systems.
>
> Chris

That's why we define atomic_read like so:

#define atomic_read(v)          ((v)->counter)

This avoids the aliasing problem, because the compiler must de-reference the 
pointer every time, which requires a memory fetch.  This is usually fast thanks 
to caching, and hardware cache invalidation enforces correctness when it does 
change.

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