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

Chris Snook
2007-08-08T06:48:26Z

Chris Friesen wrote:
> Chris Snook wrote:
>
>> This is not a problem, since indirect references will cause the CPU to
>> fetch the data from memory/cache anyway.
>
> Isn't Zan's sample code (that shows the problem) already using indirect
> references?

Yeah, I misinterpreted his conclusion. I thought about this for a
while, and realized that it's perfectly legal for the compiler to re-use
a value obtained from atomic_read. All that matters is that the read
itself was atomic. The use (or non-use) of the volatile keyword is
really more relevant to the other atomic operations. If you want to
guarantee a re-read from memory, use barrier(). This, incidentally,
uses volatile under the hood.

-- Chris