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

Chengming Zhou
2022-08-23T05:15:20Z

On 2022/8/20 08:05, Tejun Heo wrote:
> 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.

Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>

Thanks.

>
> 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);
>