Re: Difference between atomic operations and memory barriers

[Leonidas .]

On Tue, Oct 27, 2009 at 12:30 AM, Stefan Richter
<stefanr@xxxxxxxxxxxxxxxxx> wrote:
> Leonidas . wrote:
>> On Mon, Oct 26, 2009 at 11:49 PM, Noah Watkins <noah@xxxxxxxxxxxx> wrote:
>>>> Trying to understand difference between atomic ops and memory barriers.
>
> Atomic accesses and barriers are different concepts.
>
> The former means that there won't be intermediary values visible (to
> another CPU or to a DMA capable device) at any time; there will only be
> either the old value or the new value, but nothing half-done.
>
> The latter means that an order between two (or more) separate accesses
> is enforced.
>
> You cannot use barriers to make intrinsically non-atomic accesses look
> atomically; you need a lock for such a purpose.  (Or RCU.)
>
>>>> I was thinking all atomic operations must be using barrier internally, but I read
>>>> somewhere that only some of them use barriers. Sorry for being vague here.
>>>>
>>>> Operations before call to smp_mb() will not be re-ordered and all cpus will see
>>>> consistent value after the variable is updated.
>>>>
>>>> E.g.
>>>> void * ptr = (void *) str;
>>>> smb_mb();
>>>>
>>>> Will this not atomically update ptr? Ptr will be seen by all cpu's in same state
>>>> after it has been assigned str, right?
>>>
>>> There is a bit of info in:
>>>    Documentation/atomic_ops.txt
>>>
>>> -noah
>>>
>>>
>>
>>
>> Thanks for the pointer, there are no atomic operations for pointers, right?
>> I guess barrier is what we need in that case.
>
> There are more atomic accesses than those which deal with atomic_t.  For
> example,
>        any_t *ptr = something;
> is definitely atomic, and we rely on this atomicity in the kernel at
> many places.
>
> I guess these things can be found somewhere in the C language specification.
> --
> Stefan Richter
> -=====-==--= =-=- ==-=-
> http://arcgraph.de/sr/
>

If your answer matches my inference that,

any_t *ptr = something;

is always atomic even on SMPs without using locks, barriers then my
doubt is cleared. Thanks.

-Leo.
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