From: Nick Piggin <npiggin@suse.de>

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>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/readahead.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff -puN mm/readahead.c~mm-readahead-scan-lockless mm/readahead.c
--- a/mm/readahead.c~mm-readahead-scan-lockless
+++ a/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;
_