radix_tree_next_hole is implemented as a series of radix_tree_lookup()s. So
it can be called locklessly, under rcu_read_lock().

Signed-off-by: Nick Piggin <npiggin@suse.de>
---
Index: linux-2.6/mm/readahead.c
===================================================================
--- linux-2.6.orig/mm/readahead.c
+++ linux-2.6/mm/readahead.c
@@ -382,9 +382,9 @@ ondemand_readahead(struct address_space 
 	if (hit_readahead_marker) {
 		pgoff_t start;
 
-		read_lock_irq(&mapping->tree_lock);
-		start = radix_tree_next_hole(&mapping->page_tree, offset, max+1);
-		read_unlock_irq(&mapping->tree_lock);
+		rcu_read_lock();
+		start = radix_tree_next_hole(&mapping->page_tree, offset,max+1);
+		rcu_read_unlock();
 
 		if (!start || start - offset > max)
 			return 0;

-- 

--
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