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 01:23:06PM +1000, Paul Mackerras wrote:
>
> In particular, atomic_read seems to lend itself to buggy uses.  People
> seem to do things like:
> 
> 	atomic_add(&v, something);
> 	if (atomic_read(&v) > something_else) ...

If you're referring to the code in sk_stream_mem_schedule
then it's working as intended.  The atomicity guarantees
that the atomic_add/atomic_sub won't be seen in parts by
other readers.

We certainly do not need to see other atomic_add/atomic_sub
operations immediately.

If you're referring to another code snippet please cite.

> I'd go so far as to say that anywhere where you want a non-"volatile"
> atomic_read, either your code is buggy, or else an int would work just
> as well.

An int won't work here because += and -= do not have the
atomicity guarantees that atomic_add/atomic_sub do.  In
particular, this may cause an atomic_read on another CPU
to give a bogus reading.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
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