Re: Fix unlock_buffer() to work the same way as bit_unlock()

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

Chen, Kenneth W wrote:

> Nick Piggin wrote on Tuesday, March 28, 2006 12:11 AM
>
> > OK, that's fair enough and I guess you do need a barrier there.
> > However, should the mb__after barrier still remain? The comment
> > in wake_up_bit suggests yes, and there is similar code in
> > unlock_page.
>
> Question on unlock_page:
>
> void fastcall unlock_page(struct page *page)
> {
>        smp_mb__before_clear_bit();
>        if (!TestClearPageLocked(page))
>                BUG();
>        smp_mb__after_clear_bit();
>        wake_up_page(page, PG_locked);
> }
>
> Assuming test_and_clear_bit() on all arch does what the API is
> called for with full memory fence around the atomic op, why do
> you need smp_mb__before_clear_bit and smp_mb__after_clear_bit?
> Aren't they redundant?

Yep. I pointed this out earlier.

I'd say it may have initially just been a ClearPageLocked, and
was changed for debugging reasons.

We could instead change it to

BUG_ON(!PageLocked(page);
ClearPageLocked(page); /* this does clear_bit_for_unlock */
smp_mb__after_clear_bit_unlock();
wake_up_page

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