linux-kernel - [PATCH 2/2] radix-tree: Fix optimisation problem

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Date:   Thu, 22 Sep 2016 11:53:35 -0700
From:   Matthew Wilcox <mawilcox@...uxonhyperv.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        Matthew Wilcox <mawilcox@...rosoft.com>
Subject: [PATCH 2/2] radix-tree: Fix optimisation problem

From: Matthew Wilcox <mawilcox@...rosoft.com>

When compiling the radix tree with -O2, GCC thinks it can optimise:

	void *entry = parent->slots[offset];
	int siboff = entry - parent->slots;
	void *slot = parent->slots + siboff;

into

	void *slot = entry;

Unfortunately, 'entry' is a tagged pointer, so this optimisation leads
to getting an unaligned pointer back from radix_tree_lookup_slot().
The test suite wasn't being compiled with optimisation, so we hadn't
spotted it before now.  Change the test suite to compile with -O2, and
fix the optimisation problem by passing 'entry' through entry_to_node()
so gcc knows this isn't a plain pointer.
---
 lib/radix-tree.c                  | 3 ++-
 tools/testing/radix-tree/Makefile | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index 1b7bf73..8bf1f32 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -105,7 +105,8 @@ static unsigned int radix_tree_descend(struct radix_tree_node *parent,
 
 #ifdef CONFIG_RADIX_TREE_MULTIORDER
 	if (radix_tree_is_internal_node(entry)) {
-		unsigned long siboff = get_slot_offset(parent, entry);
+		unsigned long siboff = get_slot_offset(parent,
+						(void **)entry_to_node(entry));
 		if (siboff < RADIX_TREE_MAP_SIZE) {
 			offset = siboff;
 			entry = rcu_dereference_raw(parent->slots[offset]);
diff --git a/tools/testing/radix-tree/Makefile b/tools/testing/radix-tree/Makefile
index 3b53046..9d0919ed 100644
--- a/tools/testing/radix-tree/Makefile
+++ b/tools/testing/radix-tree/Makefile
@@ -1,5 +1,5 @@
 
-CFLAGS += -I. -g -Wall -D_LGPL_SOURCE
+CFLAGS += -I. -g -O2 -Wall -D_LGPL_SOURCE
 LDFLAGS += -lpthread -lurcu
 TARGETS = main
 OFILES = main.o radix-tree.o linux.o test.o tag_check.o find_next_bit.o \
-- 
2.9.3