[PATCH 1/6] fs: add namei support for doing a non-blocking path lookup

[Jens Axboe <axboe@kernel.dk>]

If the fast lookup fails, then return -EAGAIN to have the caller retry
the path lookup. Assume that a dentry having any of:

->d_revalidate()
->d_automount()
->d_manage()

could block in those callbacks. Preemptively return -EAGAIN if any of
these are present.

This is in preparation for supporting non-blocking open.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/namei.c            | 21 ++++++++++++++++++++-
 include/linux/namei.h |  2 ++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index b367fdb91682..ed108a41634f 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1641,6 +1641,17 @@ static struct dentry *__lookup_hash(const struct qstr *name,
 	return dentry;
 }
 
+static inline bool lookup_could_block(struct dentry *dentry, unsigned int flags)
+{
+	const struct dentry_operations *ops = dentry->d_op;
+
+	if (!ops || !(flags & LOOKUP_NONBLOCK))
+		return 0;
+
+	/* assume these dentry ops may block */
+	return ops->d_revalidate || ops->d_automount || ops->d_manage;
+}
+
 static int lookup_fast(struct nameidata *nd,
 		       struct path *path, struct inode **inode,
 		       unsigned *seqp)
@@ -1665,6 +1676,9 @@ static int lookup_fast(struct nameidata *nd,
 			return 0;
 		}
 
+		if (unlikely(lookup_could_block(dentry, nd->flags)))
+			return -EAGAIN;
+
 		/*
 		 * This sequence count validates that the inode matches
 		 * the dentry name information from lookup.
@@ -1707,7 +1721,10 @@ static int lookup_fast(struct nameidata *nd,
 		dentry = __d_lookup(parent, &nd->last);
 		if (unlikely(!dentry))
 			return 0;
-		status = d_revalidate(dentry, nd->flags);
+		if (unlikely(lookup_could_block(dentry, nd->flags)))
+			status = -EAGAIN;
+		else
+			status = d_revalidate(dentry, nd->flags);
 	}
 	if (unlikely(status <= 0)) {
 		if (!status)
@@ -1912,6 +1929,8 @@ static int walk_component(struct nameidata *nd, int flags)
 	if (unlikely(err <= 0)) {
 		if (err < 0)
 			return err;
+		if (nd->flags & LOOKUP_NONBLOCK)
+			return -EAGAIN;
 		path.dentry = lookup_slow(&nd->last, nd->path.dentry,
 					  nd->flags);
 		if (IS_ERR(path.dentry))
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 4e77068f7a1a..392eb439f88b 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -49,6 +49,8 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 /* LOOKUP_* flags which do scope-related checks based on the dirfd. */
 #define LOOKUP_IS_SCOPED (LOOKUP_BENEATH | LOOKUP_IN_ROOT)
 
+#define LOOKUP_NONBLOCK		0x200000 /* don't block for lookup */
+
 extern int path_pts(struct path *path);
 
 extern int user_path_at_empty(int, const char __user *, unsigned, struct path *, int *empty);
-- 
2.24.1