linux-kernel - [PATCH 08/13] VFS/namei: enhance follow

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Date:	Mon, 16 Mar 2015 15:43:20 +1100
From:	NeilBrown <neilb@...e.de>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 08/13] VFS/namei: enhance follow_link to support RCU-walk.

If LOOKUP_RCU is set, follow_link will not take/drop reference counts.

Replace cond_resched() with _cond_resched() as the latter
is a no-op if rcu_read_lock() is held while the former will
give a warning in that case.

Signed-off-by: NeilBrown <neilb@...e.de>
---
 fs/namei.c |   19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 1663d21a3eb4..536e0254f5f1 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -906,7 +906,8 @@ follow_link(struct path *link, struct nameidata *nd, void **p)
 	if (unlikely(current->nameidata->total_link_count >= 40))
 		goto out_put_nd_path;
 
-	cond_resched();
+	/* If rcu_read_locked(), this will not resched, and will not warn */
+	_cond_resched();
 	current->nameidata->total_link_count++;
 
 	if (nd->flags & LOOKUP_RCU) {
@@ -936,11 +937,17 @@ follow_link(struct path *link, struct nameidata *nd, void **p)
 			return PTR_ERR(s);
 		}
 		if (*s == '/') {
-			if (!nd->root.mnt)
-				set_root(nd);
-			path_put(&nd->path);
-			nd->path = nd->root;
-			path_get(&nd->root);
+			if (nd->flags & LOOKUP_RCU) {
+				if (!nd->root.mnt)
+					set_root_rcu(nd);
+				nd->path = nd->root;
+			} else {
+				if (!nd->root.mnt)
+					set_root(nd);
+				path_put(&nd->path);
+				nd->path = nd->root;
+				path_get(&nd->root);
+			}
 			nd->flags |= LOOKUP_JUMPED;
 		}
 		nd->inode = nd->path.dentry->d_inode;


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