[tip:core/rcu] fs/dcache: Use release-acquire for name/length update

["tip-bot for Paul E. McKenney" <tipbot@zytor.com>]

Commit-ID:  7088efa9137a15d7d21e3abce73e40c9c8a18d68
Gitweb:     https://git.kernel.org/tip/7088efa9137a15d7d21e3abce73e40c9c8a18d68
Author:     Paul E. McKenney <paulmck@linux.vnet.ibm.com>
AuthorDate: Mon, 9 Oct 2017 10:04:27 -0700
Committer:  Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CommitDate: Mon, 4 Dec 2017 10:52:52 -0800

fs/dcache: Use release-acquire for name/length update

The code in __d_alloc() carefully orders filling in the NUL character
of the name (and the length, hash, and the name itself) with assigning
of the name itself.  However, prepend_name() does not order the accesses
to the ->name and ->len fields, other than on TSO systems.  This commit
therefore replaces prepend_name()'s READ_ONCE() of ->name with an
smp_load_acquire(), which orders against the subsequent READ_ONCE() of
->len.  Because READ_ONCE() now incorporates smp_read_barrier_depends(),
prepend_name()'s smp_read_barrier_depends() is removed.  Finally,
to save a line, the smp_wmb()/store pair in __d_alloc() is replaced
by smp_store_release().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: <linux-fsdevel@vger.kernel.org>
---
 fs/dcache.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 5c7df1d..379dce8 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1636,8 +1636,7 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
 	dname[name->len] = 0;
 
 	/* Make sure we always see the terminating NUL character */
-	smp_wmb();
-	dentry->d_name.name = dname;
+	smp_store_release(&dentry->d_name.name, dname); /* ^^^ */
 
 	dentry->d_lockref.count = 1;
 	dentry->d_flags = 0;
@@ -3047,17 +3046,14 @@ static int prepend(char **buffer, int *buflen, const char *str, int namelen)
  * retry it again when a d_move() does happen. So any garbage in the buffer
  * due to mismatched pointer and length will be discarded.
  *
- * Data dependency barrier is needed to make sure that we see that terminating
- * NUL.  Alpha strikes again, film at 11...
+ * Load acquire is needed to make sure that we see that terminating NUL.
  */
 static int prepend_name(char **buffer, int *buflen, const struct qstr *name)
 {
-	const char *dname = READ_ONCE(name->name);
+	const char *dname = smp_load_acquire(&name->name); /* ^^^ */
 	u32 dlen = READ_ONCE(name->len);
 	char *p;
 
-	smp_read_barrier_depends();
-
 	*buflen -= dlen + 1;
 	if (*buflen < 0)
 		return -ENAMETOOLONG;