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

[Stefan Richter <stefanr@s5r6.in-berlin.de>]

I wrote:
> Herbert Xu wrote:
>> On Thu, Aug 16, 2007 at 10:06:31AM +0200, Stefan Richter wrote:
[...]
>>> expectation that each
>>> occurrence of atomic_read actually results in a load operation, i.e. is
>>> not optimized away.
[...]
>> Can you find an actual atomic_read code snippet there that is
>> broken without the volatile modifier?

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*.

The only sensible read accessor to an atomic datatype is a read accessor
that will not be optimized away.

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

PPS:  If somebody has code where he can afford to let the compiler
coalesce atomic_read with a previous access to the same data, i.e.
doesn't need and doesn't want all guarantees that the atomic_read API
makes (or IMO should make), then he can replace the atomic_read by a
local temporary variable.


*) Exceptions:
	if (known_to_be_false)
		read_access(a);
and the like.
-- 
Stefan Richter
-=====-=-=== =--- =----
http://arcgraph.de/sr/
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