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);
> }
-
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