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[PATCH] sync_page() smp_mb() comment 

The smp_mb() is becaus sync_page() doesn't have PG_locked while it accesses
page_mapping(page).  The comments in the patch (the entire patch is the
addition of this comment) try to explain further how and why smp_mb() is
used.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
William Lee Irwin III 2005-05-01 08:58:38 -07:00 committed by Linus Torvalds
parent 93ea1d0a12
commit dd1d5afca8
1 changed files with 19 additions and 1 deletions
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20
mm/filemap.c
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@ -139,7 +139,25 @@ static int sync_page(void *word)
page = container_of((page_flags_t *)word, struct page, flags);
/*
* FIXME, fercrissake. What is this barrier here for?
* page_mapping() is being called without PG_locked held.
* Some knowledge of the state and use of the page is used to
* reduce the requirements down to a memory barrier.
* The danger here is of a stale page_mapping() return value
* indicating a struct address_space different from the one it's
* associated with when it is associated with one.
* After smp_mb(), it's either the correct page_mapping() for
* the page, or an old page_mapping() and the page's own
* page_mapping() has gone NULL.
* The ->sync_page() address_space operation must tolerate
* page_mapping() going NULL. By an amazing coincidence,
* this comes about because none of the users of the page
* in the ->sync_page() methods make essential use of the
* page_mapping(), merely passing the page down to the backing
* device's unplug functions when it's non-NULL, which in turn
* ignore it for all cases but swap, where only page->private is
* of interest. When page_mapping() does go NULL, the entire
* call stack gracefully ignores the page and returns.
* -- wli
*/
smp_mb();
mapping = page_mapping(page);