[PATCH v3] ext4: Enable code path when DX_DEBUG is set

[Vinicius Tinti <viniciustinti@gmail.com>]

Clang with -Wunreachable-code-aggressive is being used to try to find
unreachable code that could cause potential bugs. There is no plan to
enable it by default.

The following code was detected as unreachable:

fs/ext4/namei.c:831:17: warning: code will never be executed
[-Wunreachable-code]
                        unsigned n = count - 1;
                                     ^~~~~
fs/ext4/namei.c:830:7: note: silence by adding parentheses to mark code as
explicitly dead
                if (0) { // linear search cross check
                    ^
                    /* DISABLES CODE */ ( )

This has been present since commit ac27a0ec112a ("[PATCH] ext4: initial
copy of files from ext3") and fs/ext3 had it present at the beginning of
git history. It has not been changed since.

This patch moves the code to a new function `htree_rep_invariant_check`
which only performs the check when DX_DEBUG is set. This allows the
function to be used in other parts of the code.

Suggestions from: Andreas, Christoph, Nathan, Nick and Ted.

Signed-off-by: Vinicius Tinti <viniciustinti@gmail.com>
---
 fs/ext4/namei.c | 38 ++++++++++++++++++++++++--------------
 1 file changed, 24 insertions(+), 14 deletions(-)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index cf652ba3e74d..a6e28b4b5a95 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -731,6 +731,29 @@ struct stats dx_show_entries(struct dx_hash_info *hinfo, struct inode *dir,
 		       (space/bcount)*100/blocksize);
 	return (struct stats) { names, space, bcount};
 }
+
+/*
+ * Linear search cross check
+ */
+static inline void htree_rep_invariant_check(struct dx_entry *at,
+					     struct dx_entry *target,
+					     u32 hash, unsigned int n)
+{
+	while (n--) {
+		dxtrace(printk(KERN_CONT ","));
+		if (dx_get_hash(++at) > hash) {
+			at--;
+			break;
+		}
+	}
+	ASSERT(at == target - 1);
+}
+#else /* DX_DEBUG */
+static inline void htree_rep_invariant_check(struct dx_entry *at,
+					     struct dx_entry *target,
+					     u32 hash, unsigned int n)
+{
+}
 #endif /* DX_DEBUG */
 
 /*
@@ -827,20 +850,7 @@ dx_probe(struct ext4_filename *fname, struct inode *dir,
 				p = m + 1;
 		}
 
-		if (0) { // linear search cross check
-			unsigned n = count - 1;
-			at = entries;
-			while (n--)
-			{
-				dxtrace(printk(KERN_CONT ","));
-				if (dx_get_hash(++at) > hash)
-				{
-					at--;
-					break;
-				}
-			}
-			ASSERT(at == p - 1);
-		}
+		htree_rep_invariant_check(entries, p, hash, count - 1);
 
 		at = p - 1;
 		dxtrace(printk(KERN_CONT " %x->%u\n",
-- 
2.25.1