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

Chris Snook
2007-08-09T12:53:52Z

Robert P. J. Day wrote:
> On Wed, 8 Aug 2007, Chris Snook wrote:
>
>> Jerry Jiang wrote:
>>> On Wed, 08 Aug 2007 02:47:53 -0400
>>> Chris Snook <csnook@redhat.com> wrote:
>>>
>>>> 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.
>>>>
>>>
>>> So for example, without volatile
>>>
>>> int a = read_atomic(v);
>>> int b = read_atomic(v);
>>>
>>> the compiler will optimize it as b = a, But with volatile, it will be forced
>>> to fetch v's value from memory
>>> again.
>>>
>>> So, come back our initial question,
>>> include/asm-v850/atomic.h:typedef struct { int counter; } atomic_t;
>>>
>>> Why is it right without volatile?
>> Because atomic_t doesn't promise a memory fetch every time. It merely
>> promises that any atomic_* operations will, in fact, be atomic. For example,
>> posted today:
>>
>> http://lkml.org/lkml/2007/8/8/122
>
> i'm sure that, when this is all done, i'll finally have an answer to
> my original question, "why are some atomic_t's not volatile, while
> most are?"
>
> i'm almost scared to ask any more questions. :-)
>
> rday

Momentarily I'll be posting a patchset that makes all atomic_t and atomic64_t
declarations non-volatile, and casts them to volatile inside of atomic[64]_read.
This will ensure consistent behavior across all architectures, and is in
keeping with the philosophy that memory reads should be enforced in running
code, not declarations.

I hope you don't mind that we're mooting the question by making the code more
sensible.

-- Chris