linux-kernel - [PATCH 4/9] VFS/namei: abort RCU-walk on symlink if atime needs updating.

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Date:	Thu, 05 Mar 2015 16:21:21 +1100
From:	NeilBrown <neilb@...e.de>
To:	Alexander Viro <viro@...iv.linux.org.uk>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 4/9] VFS/namei: abort RCU-walk on symlink if atime needs
 updating.

touch_atime is not RCU-safe, and so cannot be called on an
RCU walk.
However in situations where RCU-walk makes a difference,
the symlink will likely to accessed much more often than
it is useful to update the atime.

So split out the test of "Does the atime actually need to be updated"
into  atime_needs_update(), and only allow RCU-walk on a symlink if
that fails.

Signed-off-by: NeilBrown <neilb@...e.de>
---
 fs/inode.c         |   26 +++++++++++++++++++-------
 fs/namei.c         |    7 ++++++-
 include/linux/fs.h |    1 +
 3 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index f00b16f45507..a0da920e4650 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1584,30 +1584,41 @@ static int update_time(struct inode *inode, struct timespec *time, int flags)
  *	This function automatically handles read only file systems and media,
  *	as well as the "noatime" flag and inode specific "noatime" markers.
  */
-void touch_atime(const struct path *path)
+int atime_needs_update(const struct path *path)
 {
 	struct vfsmount *mnt = path->mnt;
 	struct inode *inode = path->dentry->d_inode;
 	struct timespec now;
 
 	if (inode->i_flags & S_NOATIME)
-		return;
+		return 0;
 	if (IS_NOATIME(inode))
-		return;
+		return 0;
 	if ((inode->i_sb->s_flags & MS_NODIRATIME) && S_ISDIR(inode->i_mode))
-		return;
+		return 0;
 
 	if (mnt->mnt_flags & MNT_NOATIME)
-		return;
+		return 0;
 	if ((mnt->mnt_flags & MNT_NODIRATIME) && S_ISDIR(inode->i_mode))
-		return;
+		return 0;
 
 	now = current_fs_time(inode->i_sb);
 
 	if (!relatime_need_update(mnt, inode, now))
-		return;
+		return 0;
 
 	if (timespec_equal(&inode->i_atime, &now))
+		return 0;
+	return 1;
+}
+
+void touch_atime(const struct path *path)
+{
+	struct vfsmount *mnt = path->mnt;
+	struct inode *inode = path->dentry->d_inode;
+	struct timespec now;
+
+	if (!atime_needs_update(path))
 		return;
 
 	if (!sb_start_write_trylock(inode->i_sb))
@@ -1624,6 +1635,7 @@ void touch_atime(const struct path *path)
 	 * We may also fail on filesystems that have the ability to make parts
 	 * of the fs read only, e.g. subvolumes in Btrfs.
 	 */
+	now = current_fs_time(inode->i_sb);
 	update_time(inode, &now, S_ATIME);
 	__mnt_drop_write(mnt);
 skip_update:
diff --git a/fs/namei.c b/fs/namei.c
index a4879bb3b4ee..5814d1b2daab 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -886,7 +886,12 @@ follow_link(struct path *link, struct nameidata *nd, void **p)
 	cond_resched();
 	current->total_link_count++;
 
-	touch_atime(link);
+	if (nd->flags & LOOKUP_RCU) {
+		error = -ECHILD;
+		if (atime_needs_update(link))
+			goto out_put_nd_path;
+	} else
+		touch_atime(link);
 	nd_set_link(nd, NULL);
 
 	error = security_inode_follow_link(link->dentry, nd);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index eaef987ae3cf..6665aa16abcb 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1845,6 +1845,7 @@ enum file_time_flags {
 	S_VERSION = 8,
 };
 
+extern int atime_needs_update(const struct path *);
 extern void touch_atime(const struct path *);
 static inline void file_accessed(struct file *file)
 {


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