On Fri, 17 Aug 2007, Nick Piggin wrote:

Satyam Sharma wrote:

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On Fri, 17 Aug 2007, Nick Piggin wrote:

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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? :-)