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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