Re: [PATCH] Keys: Improve usage of memory barriers and remove IRQ disablement

[Nick Piggin <nickpiggin@yahoo.com.au>]

David Howells wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> | 	int atomic_inc_and_test(atomic_t *v);
> | 	int atomic_dec_and_test(atomic_t *v);
> | 
> | These two routines increment and decrement by 1, respectively, the
> | given atomic counter.  They return a boolean indicating whether the
> | resulting counter value was zero or not.
> | 
> | It requires explicit memory barrier semantics around the operation as
> | above.
>
> Note the last paragraph.  "It requires" should be "They require", but the
> sense would seem to be obvious.  However, it's not clear on a second reading
> as to whether this is an instruction to the _caller_ or an instruction to the
> arch _implementer_.

Yes, I remember Dave M clarified this sometime ago (on lkml I guess). It
is a little confusing, but I think the wording is for the implementer's
point of view. Dave will pull me up if I'm wrong...

> I suppose from reading the abstract at the top:
>
> | This document is intended to serve as a guide to Linux port maintainers on
> | how to implement atomic counter, bitops, and spinlock interfaces properly.
>
> that it is meant to be read by the implementor and not the user/caller, in which
> case, Nick is correct.
>
> It seems I need to adjust my memory barrier doc, and perhaps I should adjust
> atomic_ops.txt too to make it clearer.

I think that would be good. atomic_ops.txt is very useful for API users
as well, so if it can be made more general without becoming ambiguous,
I'm sure that would be appreciated.

Thanks,
Nick

--
SUSE Labs, Novell Inc.
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