[PATCH 1/7] kernfs: Simply by replacing kernfs_deref_open_node() with of_on()

Tejun Heo
2022-08-20T00:06:17Z

kernfs_node->attr.open is an RCU pointer to kernfs_open_node. However, RCU
dereference is currently only used in kernfs_notify(). Everywhere else,
either we're holding the lock which protects it or know that the
kernfs_open_node is pinned becaused we have a pointer to a kernfs_open_file
which is hanging off of it.

kernfs_deref_open_node() is used for the latter case - accessing
kernfs_open_node from kernfs_open_file. The lifetime and visibility rules
are simple and clear here. To someone who can access a kernfs_open_file, its
kernfs_open_node is pinned and visible through of->kn->attr.open.

Replace kernfs_deref_open_node() which simpler of_on(). The former takes
both @kn and @of and RCU deref @kn->attr.open while sanity checking with
@of. The latter takes @of and uses protected deref on of->kn->attr.open.

As the return value can't be NULL, remove the error handling in the callers
too.

This shouldn't cause any functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Imran Khan <imran.f.khan@oracle.com>
---
fs/kernfs/file.c | 56 +++++++++++-------------------------------------
1 file changed, 13 insertions(+), 43 deletions(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index b3ec34386b43..32b16fe00a9e 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -57,31 +57,17 @@ static inline struct mutex *kernfs_open_file_mutex_lock(struct kernfs_node *kn)
}

/**
- * kernfs_deref_open_node - Get kernfs_open_node corresponding to @kn.
- *
- * @of: associated kernfs_open_file instance.
- * @kn: target kernfs_node.
- *
- * Fetch and return ->attr.open of @kn if @of->list is non empty.
- * If @of->list is not empty we can safely assume that @of is on
- * @kn->attr.open->files list and this guarantees that @kn->attr.open
- * will not vanish i.e. dereferencing outside RCU read-side critical
- * section is safe here.
- *
- * The caller needs to make sure that @of->list is not empty.
+ * of_on - Return the kernfs_open_node of the specified kernfs_open_file
+ * @of: taret kernfs_open_file
*/
-static struct kernfs_open_node *
-kernfs_deref_open_node(struct kernfs_open_file *of, struct kernfs_node *kn)
+static struct kernfs_open_node *of_on(struct kernfs_open_file *of)
{
- struct kernfs_open_node *on;
-
- on = rcu_dereference_check(kn->attr.open, !list_empty(&of->list));
-
- return on;
+ return rcu_dereference_protected(of->kn->attr.open,
+ !list_empty(&of->list));
}

/**
- * kernfs_deref_open_node_protected - Get kernfs_open_node corresponding to @kn
+ * kernfs_deref_open_node_locked - Get kernfs_open_node corresponding to @kn
*
* @kn: target kernfs_node.
*
@@ -96,7 +82,7 @@ kernfs_deref_open_node(struct kernfs_open_file *of, struct kernfs_node *kn)
* The caller needs to make sure that kernfs_open_file_mutex is held.
*/
static struct kernfs_open_node *
-kernfs_deref_open_node_protected(struct kernfs_node *kn)
+kernfs_deref_open_node_locked(struct kernfs_node *kn)
{
return rcu_dereference_protected(kn->attr.open,
lockdep_is_held(kernfs_open_file_mutex_ptr(kn)));
@@ -207,12 +193,8 @@ static void kernfs_seq_stop(struct seq_file *sf, void *v)
static int kernfs_seq_show(struct seq_file *sf, void *v)
{
struct kernfs_open_file *of = sf->private;
- struct kernfs_open_node *on = kernfs_deref_open_node(of, of->kn);
-
- if (!on)
- return -EINVAL;

- of->event = atomic_read(&on->event);
+ of->event = atomic_read(&of_on(of)->event);

return of->kn->attr.ops->seq_show(sf, v);
}
@@ -235,7 +217,6 @@ static ssize_t kernfs_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
struct kernfs_open_file *of = kernfs_of(iocb->ki_filp);
ssize_t len = min_t(size_t, iov_iter_count(iter), PAGE_SIZE);
const struct kernfs_ops *ops;
- struct kernfs_open_node *on;
char *buf;

buf = of->prealloc_buf;
@@ -257,14 +238,7 @@ static ssize_t kernfs_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
goto out_free;
}

- on = kernfs_deref_open_node(of, of->kn);
- if (!on) {
- len = -EINVAL;
- mutex_unlock(&of->mutex);
- goto out_free;
- }
-
- of->event = atomic_read(&on->event);
+ of->event = atomic_read(&of_on(of)->event);

ops = kernfs_ops(of->kn);
if (ops->read)
@@ -584,7 +558,7 @@ static int kernfs_get_open_node(struct kernfs_node *kn,
struct mutex *mutex = NULL;

mutex = kernfs_open_file_mutex_lock(kn);
- on = kernfs_deref_open_node_protected(kn);
+ on = kernfs_deref_open_node_locked(kn);

if (on) {
list_add_tail(&of->list, &on->files);
@@ -629,7 +603,7 @@ static void kernfs_unlink_open_file(struct kernfs_node *kn,

mutex = kernfs_open_file_mutex_lock(kn);

- on = kernfs_deref_open_node_protected(kn);
+ on = kernfs_deref_open_node_locked(kn);
if (!on) {
mutex_unlock(mutex);
return;
@@ -839,7 +813,7 @@ void kernfs_drain_open_files(struct kernfs_node *kn)
return;

mutex = kernfs_open_file_mutex_lock(kn);
- on = kernfs_deref_open_node_protected(kn);
+ on = kernfs_deref_open_node_locked(kn);
if (!on) {
mutex_unlock(mutex);
return;
@@ -874,11 +848,7 @@ void kernfs_drain_open_files(struct kernfs_node *kn)
*/
__poll_t kernfs_generic_poll(struct kernfs_open_file *of, poll_table *wait)
{
- struct kernfs_node *kn = kernfs_dentry_node(of->file->f_path.dentry);
- struct kernfs_open_node *on = kernfs_deref_open_node(of, kn);
-
- if (!on)
- return EPOLLERR;
+ struct kernfs_open_node *on = of_on(of);

poll_wait(of->file, &on->poll, wait);

--
2.37.2