Messages in this thread | | | Date | Thu, 3 May 2007 13:24:23 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: 2.6.22 -mm merge plans -- vm bugfixes |
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On Thu, 3 May 2007, Nick Piggin wrote: > > The problem is that lock/unlock_page is expensive on powerpc, and > if we improve that, we improve more than just the fault handler... > > The attached patch gets performance up a bit by avoiding some > barriers and some cachelines:
There's a strong whiff of raciness about this... but I could very easily be wrong.
> Index: linux-2.6/mm/filemap.c > =================================================================== > --- linux-2.6.orig/mm/filemap.c 2007-05-02 15:00:26.000000000 +1000 > +++ linux-2.6/mm/filemap.c 2007-05-03 08:34:32.000000000 +1000 > @@ -532,11 +532,13 @@ > */ > void fastcall unlock_page(struct page *page) > { > + VM_BUG_ON(!PageLocked(page)); > smp_mb__before_clear_bit(); > - if (!TestClearPageLocked(page)) > - BUG(); > - smp_mb__after_clear_bit(); > - wake_up_page(page, PG_locked); > + ClearPageLocked(page); > + if (unlikely(test_bit(PG_waiters, &page->flags))) { > + clear_bit(PG_waiters, &page->flags); > + wake_up_page(page, PG_locked); > + } > } > EXPORT_SYMBOL(unlock_page); > > @@ -568,6 +570,11 @@ __lock_page (diff -p would tell us!) > { > DEFINE_WAIT_BIT(wait, &page->flags, PG_locked); > > + set_bit(PG_waiters, &page->flags); > + if (unlikely(!TestSetPageLocked(page))) {
What happens if another cpu is coming through __lock_page at the same time, did its set_bit, now finds PageLocked, and so proceeds to the __wait_on_bit_lock? But this cpu now clears PG_waiters, so this task's unlock_page won't wake the other?
> + clear_bit(PG_waiters, &page->flags); > + return; > + } > __wait_on_bit_lock(page_waitqueue(page), &wait, sync_page, > TASK_UNINTERRUPTIBLE); > } - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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