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SubjectUnnecessary barrier in sync_page()?

Hi Chris,

I was talking to Andrew about this memory barrier

static inline int sync_page(struct page *page)
{
struct address_space *mapping;

/*
* FIXME, fercrissake. What is this barrier here for?
*/
smp_mb();
mapping = page_mapping(page);
if (mapping && mapping->a_ops && mapping->a_ops->sync_page)
return mapping->a_ops->sync_page(page);
return 0;
}

And does not seem to be a reason for it. The callers are:

void fastcall wait_on_page_bit(struct page *page, int bit_nr)
{
wait_queue_head_t *waitqueue = page_waitqueue(page);
DEFINE_PAGE_WAIT(wait, page, bit_nr);

do {
prepare_to_wait(waitqueue, &wait.wait, TASK_UNINTERRUPTIBLE);
if (test_bit(bit_nr, &page->flags)) {
sync_page(page);
io_schedule();
}
} while (test_bit(bit_nr, &page->flags));
finish_wait(waitqueue, &wait.wait);
}

void fastcall __lock_page(struct page *page)
{
wait_queue_head_t *wqh = page_waitqueue(page);
DEFINE_PAGE_WAIT_EXCLUSIVE(wait, page, PG_locked);

while (TestSetPageLocked(page)) {
prepare_to_wait_exclusive(wqh, &wait.wait, TASK_UNINTERRUPTIBLE);
if (PageLocked(page)) {
sync_page(page);
io_schedule();
}
}
finish_wait(wqh, &wait.wait);
}

Both callers call set_bit (atomic operation which cannot be reordered) before
sync_page(), so.. what is the barrier trying to guarantee?

TIA

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