Re: [PATCH 04 of 31] update futex compound knowledge

[Andrea Arcangeli <aarcange@redhat.com>]

There was a little problem in the futex code, basically we can't take
PG_lock on a page that isn't pinned. This should fix it. I never had a
problem in practice. Untested still.. just wanted to update you on
this one.

diff --git a/kernel/futex.c b/kernel/futex.c
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -258,6 +258,18 @@ again:
 		local_irq_disable();
 		if (likely(__get_user_pages_fast(address, 1, 1, &page) == 1)) {
 			page_head = compound_head(page);
+			/*
+			 * page_head is valid pointer but we must pin
+			 * it before taking the PG_lock and/or
+			 * PG_compound_lock. The moment we re-enable
+			 * irqs __split_huge_page_splitting() can
+			 * return and the head page can be freed from
+			 * under us. We can't take the PG_lock and/or
+			 * PG_compound_lock on a page that could be
+			 * freed from under us.
+			 */
+			if (page != page_head)
+				get_page(page_head);
 			local_irq_enable();
 		} else {
 			local_irq_enable();
@@ -266,6 +278,8 @@ again:
 	}
 #else
 	page_head = compound_head(page);
+	if (page != page_head)
+		get_page(page_head);
 #endif
 
 	lock_page(page_head);
@@ -274,12 +288,15 @@ again:
 		if (unlikely(!PageTail(page))) {
 			compound_unlock(page_head);
 			unlock_page(page_head);
+			put_page(page_head);
 			put_page(page);
 			goto again;
 		}
 	}
 	if (!page_head->mapping) {
 		unlock_page(page_head);
+		if (page_head != page)
+			put_page(page_head);
 		put_page(page);
 		goto again;
 	}
@@ -303,9 +320,13 @@ again:
 
 	get_futex_key_refs(key);
 
-	if (unlikely(PageTail(page)))
+	unlock_page(page_head);
+	if (page != page_head) {
+		VM_BUG_ON(!PageTail(page));
+		/* releasing compound_lock after page_lock won't matter */
 		compound_unlock(page_head);
-	unlock_page(page_head);
+		put_page(page_head);
+	}
 	put_page(page);
 	return 0;
 }

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