[RFC][PATCH v4 54/69] non-RCU analogue of the previous commit

Al Viro
2020-03-13T23:55:10Z

new helper: choose_mountpoint(). Wrapper around choose_mountpoint_rcu(),
similar to lookup_mnt() vs. __lookup_mnt(). follow_dotdot() switched to
it. Now we don't grab mount_lock exclusive anymore; note that the
primitive used non-RCU mount traversals in other direction (lookup_mnt())
doesn't bother with that either - it uses mount_lock seqcount instead.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
fs/namei.c | 56 +++++++++++++++++++++++++++++++++++++++-----------------
1 file changed, 39 insertions(+), 17 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index caa109941788..a9622e629e76 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -605,10 +605,9 @@ static void terminate_walk(struct nameidata *nd)
}

/* path_put is needed afterwards regardless of success or failure */
-static bool legitimize_path(struct nameidata *nd,
- struct path *path, unsigned seq)
+static bool __legitimize_path(struct path *path, unsigned seq, unsigned mseq)
{
- int res = __legitimize_mnt(path->mnt, nd->m_seq);
+ int res = __legitimize_mnt(path->mnt, mseq);
if (unlikely(res)) {
if (res > 0)
path->mnt = NULL;
@@ -622,6 +621,12 @@ static bool legitimize_path(struct nameidata *nd,
return !read_seqcount_retry(&path->dentry->d_seq, seq);
}

+static inline bool legitimize_path(struct nameidata *nd,
+ struct path *path, unsigned seq)
+{
+ return __legitimize_path(path, nd->m_seq, seq);
+}
+
static bool legitimize_links(struct nameidata *nd)
{
int i;
@@ -1383,6 +1388,31 @@ static bool choose_mountpoint_rcu(struct mount *m, const struct path *root,
return false;
}

+static bool choose_mountpoint(struct mount *m, const struct path *root,
+ struct path *path)
+{
+ bool found;
+
+ rcu_read_lock();
+ while (1) {
+ unsigned seq, mseq = read_seqbegin(&mount_lock);
+
+ found = choose_mountpoint_rcu(m, root, path, &seq);
+ if (unlikely(!found)) {
+ if (!read_seqretry(&mount_lock, mseq))
+ break;
+ } else {
+ if (likely(__legitimize_path(path, seq, mseq)))
+ break;
+ rcu_read_unlock();
+ path_put(path);
+ rcu_read_lock();
+ }
+ }
+ rcu_read_unlock();
+ return found;
+}
+
/*
* Skip to top of mountpoint pile in refwalk mode for follow_dotdot()
*/
@@ -1756,22 +1786,14 @@ static struct dentry *follow_dotdot(struct nameidata *nd,
if (path_equal(&nd->path, &nd->root))
goto in_root;
if (unlikely(nd->path.dentry == nd->path.mnt->mnt_root)) {
- struct path path = nd->path;
- path_get(&path);
- while (1) {
- if (!follow_up(&path)) {
- path_put(&path);
- goto in_root;
- }
- if (path_equal(&path, &nd->root)) {
- path_put(&path);
- goto in_root;
- }
- if (path.dentry != nd->path.mnt->mnt_root)
- break;
- }
+ struct path path;
+
+ if (!choose_mountpoint(real_mount(nd->path.mnt),
+ &nd->root, &path))
+ goto in_root;
path_put(&nd->path);
nd->path = path;
+ nd->inode = path.dentry->d_inode;
if (unlikely(nd->flags & LOOKUP_NO_XDEV))
return ERR_PTR(-EXDEV);
}
--
2.11.0