linux-kernel - [RFC][PATCH v3 42/55] follow_dotdot{,_rcu}(): lift switching nd->path to parent out of loop

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Date:   Sun,  1 Mar 2020 21:52:27 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     linux-fsdevel@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [RFC][PATCH v3 42/55] follow_dotdot{,_rcu}(): lift switching nd->path to parent out of loop

From: Al Viro <viro@...iv.linux.org.uk>

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
 fs/namei.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 9b66cd00edcb..c307bf7cbaa1 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1365,7 +1365,9 @@ static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,
 
 static int follow_dotdot_rcu(struct nameidata *nd)
 {
+	struct dentry *parent = NULL;
 	struct inode *inode = nd->inode;
+	unsigned seq;
 
 	while (1) {
 		if (path_equal(&nd->path, &nd->root)) {
@@ -1375,15 +1377,12 @@ static int follow_dotdot_rcu(struct nameidata *nd)
 		}
 		if (nd->path.dentry != nd->path.mnt->mnt_root) {
 			struct dentry *old = nd->path.dentry;
-			struct dentry *parent = old->d_parent;
-			unsigned seq;
 
+			parent = old->d_parent;
 			inode = parent->d_inode;
 			seq = read_seqcount_begin(&parent->d_seq);
 			if (unlikely(read_seqcount_retry(&old->d_seq, nd->seq)))
 				return -ECHILD;
-			nd->path.dentry = parent;
-			nd->seq = seq;
 			if (unlikely(!path_connected(nd->path.mnt, parent)))
 				return -ECHILD;
 			break;
@@ -1406,6 +1405,10 @@ static int follow_dotdot_rcu(struct nameidata *nd)
 			nd->seq = seq;
 		}
 	}
+	if (likely(parent)) {
+		nd->path.dentry = parent;
+		nd->seq = seq;
+	}
 	while (unlikely(d_mountpoint(nd->path.dentry))) {
 		struct mount *mounted;
 		mounted = __lookup_mnt(nd->path.mnt, nd->path.dentry);
@@ -1442,6 +1445,7 @@ static void follow_mount(struct path *path)
 
 static int follow_dotdot(struct nameidata *nd)
 {
+	struct dentry *parent = NULL;
 	while (1) {
 		if (path_equal(&nd->path, &nd->root)) {
 			if (unlikely(nd->flags & LOOKUP_BENEATH))
@@ -1450,14 +1454,11 @@ static int follow_dotdot(struct nameidata *nd)
 		}
 		if (nd->path.dentry != nd->path.mnt->mnt_root) {
 			/* rare case of legitimate dget_parent()... */
-			struct dentry *parent = dget_parent(nd->path.dentry);
-
+			parent = dget_parent(nd->path.dentry);
 			if (unlikely(!path_connected(nd->path.mnt, parent))) {
 				dput(parent);
 				return -ENOENT;
 			}
-			dput(nd->path.dentry);
-			nd->path.dentry = parent;
 			break;
 		}
 		if (!follow_up(&nd->path))
@@ -1465,6 +1466,10 @@ static int follow_dotdot(struct nameidata *nd)
 		if (unlikely(nd->flags & LOOKUP_NO_XDEV))
 			return -EXDEV;
 	}
+	if (likely(parent)) {
+		dput(nd->path.dentry);
+		nd->path.dentry = parent;
+	}
 	follow_mount(&nd->path);
 	nd->inode = nd->path.dentry->d_inode;
 	return 0;
-- 
2.11.0