Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

[Herbert Xu <herbert@gondor.apana.org.au>]

On Thu, Aug 16, 2007 at 12:31:03PM +0200, Stefan Richter wrote:
> 
> PS:  Just to clarify, I'm not speaking for the volatile modifier.  I'm
> not speaking for any particular implementation of atomic_t and its
> accessors at all.  All I am saying is that
>   - we use atomically accessed data types because we concurrently but
>     locklessly access this data,
>   - hence a read access to this data that could be optimized away
>     makes *no sense at all*.

No sane compiler can optimise away an atomic_read per se.
That's only possible if there's a preceding atomic_set or
atomic_read, with no barriers in the middle.

If that's the case, then one has to conclude that doing
away with the second read is acceptable, as otherwise
a memory (or at least a compiler) barrier should have been
used.

In fact, volatile doesn't guarantee that the memory gets
read anyway.  You might be reading some stale value out
of the cache.  Granted this doesn't happen on x86 but
when you're coding for the kernel you can't make such
assumptions.

So the point here is that if you don't mind getting a stale
value from the CPU cache when doing an atomic_read, then
surely you won't mind getting a stale value from the compiler
"cache".

> So, the architecture guys can implement atomic_read however they want
> --- as long as it cannot be optimized away.*

They can implement it however they want as long as it stays
atomic.

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
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