Messages in this thread | | | Date | Thu, 30 Mar 2006 12:36:25 +1100 | From | Nick Piggin <> | Subject | Re: Fix unlock_buffer() to work the same way as bit_unlock() |
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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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