[PATCH 15/20] VFS/namei: enhance follow_link to support RCU-walk.

NeilBrown
2015-03-23T02:39:36Z

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.

After taking a copy of dentry->d_inode, check d_seq to ensure this
is still the symlink we were looking for.

Signed-off-by: NeilBrown <neilb@suse.de>
---
fs/namei.c | 27 +++++++++++++++++++--------
1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 72f5a4f91855..40ff4cb04244 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -897,16 +897,21 @@ follow_link(struct path *link, struct nameidata *nd, void **p)
char *s;

nd->flags &= ~LOOKUP_LINK_RCU;
- if (nd->flags & LOOKUP_RCU)
+ if (nd->flags & LOOKUP_RCU) {
nd->flags |= LOOKUP_LINK_RCU;
- else if (link->mnt == nd->path.mnt)
+ if (__read_seqcount_retry(&dentry->d_seq, nd->seq)) {
+ error = -ECHILD;
+ goto out_put_nd_path;
+ }
+ } else if (link->mnt == nd->path.mnt)
mntget(link->mnt);

error = -ELOOP;
if (unlikely(nd->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();
nd->total_link_count++;

if (nd->flags & LOOKUP_RCU) {
@@ -938,11 +943,17 @@ 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;