vfs: do the careful dentry name access for all dentry_cmp cases

Commit 12f8ad4b0533 ("vfs: clean up __d_lookup_rcu() and dentry_cmp()
interfaces") did the careful ACCESS_ONCE() of the dentry name only for
the word-at-a-time case, even though the issue is generic.

Admittedly I don't really see gcc ever reloading the value in the middle
of the loop, so the ACCESS_ONCE() protects us from a fairly theoretical
issue. But better safe than sorry.

Also, this consolidates the common parts of the word-at-a-time and
bytewise logic, which includes checking the length.  We'll be changing
that later.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/dcache.c

@@ -153,30 +153,9 @@ int proc_nr_dentry(ctl_table *table, int write, void __user *buffer,
  * In contrast, 'ct' and 'tcount' can be from a pathname, and do
  * need the careful unaligned handling.
  */
-static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *ct, unsigned tcount)
+static inline int dentry_string_cmp(const unsigned char *cs, const unsigned char *ct, unsigned tcount)
 {
 	unsigned long a,b,mask;
-	const unsigned char *cs;
-
-	if (unlikely(dentry->d_name.len != tcount))
-		return 1;
-	/*
-	 * Be careful about RCU walk racing with rename:
-	 * use ACCESS_ONCE to fetch the name pointer.
-	 *
-	 * NOTE! Even if a rename will mean that the length
-	 * was not loaded atomically, we don't care. The
-	 * RCU walk will check the sequence count eventually,
-	 * and catch it. And we won't overrun the buffer,
-	 * because we're reading the name pointer atomically,
-	 * and a dentry name is guaranteed to be properly
-	 * terminated with a NUL byte.
-	 *
-	 * End result: even if 'len' is wrong, we'll exit
-	 * early because the data cannot match (there can
-	 * be no NUL in the ct/tcount data)
-	 */
-	cs = ACCESS_ONCE(dentry->d_name.name);
 
 	for (;;) {
 		a = *(unsigned long *)cs;
@@ -197,13 +176,8 @@ static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *c
 
 #else
 
-static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *ct, unsigned tcount)
+static inline int dentry_string_cmp(const unsigned char *cs, const unsigned char *ct, unsigned tcount)
 {
-	const unsigned char *cs = dentry->d_name.name;
-
-	if (dentry->d_name.len != tcount)
-		return 1;
-
 	do {
 		if (*cs != *ct)
 			return 1;
@@ -216,6 +190,30 @@ static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *c
 
 #endif
 
+static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *ct, unsigned tcount)
+{
+	if (dentry->d_name.len != tcount)
+		return 1;
+
+	/*
+	 * Be careful about RCU walk racing with rename:
+	 * use ACCESS_ONCE to fetch the name pointer.
+	 *
+	 * NOTE! Even if a rename will mean that the length
+	 * was not loaded atomically, we don't care. The
+	 * RCU walk will check the sequence count eventually,
+	 * and catch it. And we won't overrun the buffer,
+	 * because we're reading the name pointer atomically,
+	 * and a dentry name is guaranteed to be properly
+	 * terminated with a NUL byte.
+	 *
+	 * End result: even if 'len' is wrong, we'll exit
+	 * early because the data cannot match (there can
+	 * be no NUL in the ct/tcount data)
+	 */
+	return dentry_string_cmp(ACCESS_ONCE(dentry->d_name.name), ct, tcount);
+}
+
 static void __d_free(struct rcu_head *head)
 {
 	struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);