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

Jens Axboe
2020-01-07T17:05:46Z

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