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

[Satyam Sharma <satyam@infradead.org>]

On Fri, 17 Aug 2007, Nick Piggin wrote:

> Satyam Sharma wrote:
> > 
> > On Fri, 17 Aug 2007, Nick Piggin wrote:
> 
> > > > Sure, now
> > > > that I learned of these properties I can start to audit code and insert
> > > > barriers where I believe they are needed, but this simply means that
> > > > almost all occurrences of atomic_read will get barriers (unless there
> > > > already are implicit but more or less obvious barriers like msleep).
> > > 
> > > You might find that these places that appear to need barriers are
> > > buggy for other reasons anyway. Can you point to some in-tree code
> > > we can have a look at?
> > 
> > 
> > Such code was mentioned elsewhere (query nodemgr_host_thread in cscope)
> > that managed to escape the requirement for a barrier only because of
> > some completely un-obvious compilation-unit-scope thing. But I find such
> > an non-explicit barrier quite bad taste. Stefan, do consider plunking an
> > explicit call to barrier() there.
> 
> It is very obvious. msleep calls schedule() (ie. sleeps), which is
> always a barrier.

Probably you didn't mean that, but no, schedule() is not barrier because
it sleeps. It's a barrier because it's invisible.

> The "unobvious" thing is that you wanted to know how the compiler knows
> a function is a barrier -- answer is that if it does not *know* it is not
> a barrier, it must assume it is a barrier.

True, that's clearly what happens here. But are you're definitely joking
that this is "obvious" in terms of code-clarity, right?

Just 5 minutes back you mentioned elsewhere you like seeing lots of
explicit calls to barrier() (with comments, no less, hmm? :-)
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