kernfs: Remove reference counting for kernfs_open_node.

The decision to free kernfs_open_node object in kernfs_put_open_node can
be taken based on whether kernfs_open_node->files list is empty or not. As
far as kernfs_drain_open_files is concerned it can't overlap with
kernfs_fops_open and hence can check for ->attr.open optimistically
(if ->attr.open is NULL) or under kernfs_open_file_mutex (if it needs to
traverse the ->files list.) Thus kernfs_drain_open_files can work w/o ref
counting involved kernfs_open_node as well.
So remove ->refcnt and modify the above mentioned users accordingly.

Suggested by: Al Viro <viro@zeniv.linux.org.uk>

Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
Link: https://lore.kernel.org/r/20220324103040.584491-2-imran.f.khan@oracle.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 88423069407c04ab0253d654713943cc4b09c459..aea6968c979eb42591843557f960a9be56403204 100644 (file)
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -33,7 +33,6 @@ static DEFINE_SPINLOCK(kernfs_open_node_lock);
 static DEFINE_MUTEX(kernfs_open_file_mutex);
 
 struct kernfs_open_node {
-       atomic_t                refcnt;
        atomic_t                event;
        wait_queue_head_t       poll;
        struct list_head        files; /* goes through kernfs_open_file.list */
@@ -530,10 +529,8 @@ static int kernfs_get_open_node(struct kernfs_node *kn,
        }
 
        on = kn->attr.open;
-       if (on) {
-               atomic_inc(&on->refcnt);
+       if (on)
                list_add_tail(&of->list, &on->files);
-       }
 
        spin_unlock_irq(&kernfs_open_node_lock);
        mutex_unlock(&kernfs_open_file_mutex);
@@ -548,7 +545,6 @@ static int kernfs_get_open_node(struct kernfs_node *kn,
        if (!new_on)
                return -ENOMEM;
 
-       atomic_set(&new_on->refcnt, 0);
        atomic_set(&new_on->event, 1);
        init_waitqueue_head(&new_on->poll);
        INIT_LIST_HEAD(&new_on->files);
@@ -557,11 +553,12 @@ static int kernfs_get_open_node(struct kernfs_node *kn,
 
 /**
  *     kernfs_put_open_node - put kernfs_open_node
- *     @kn: target kernfs_nodet
+ *     @kn: target kernfs_node
  *     @of: associated kernfs_open_file
  *
  *     Put @kn->attr.open and unlink @of from the files list.  If
- *     reference count reaches zero, disassociate and free it.
+ *     list of associated open files becomes empty, disassociate and
+ *     free kernfs_open_node.
  *
  *     LOCKING:
  *     None.
@@ -578,7 +575,7 @@ static void kernfs_put_open_node(struct kernfs_node *kn,
        if (of)
                list_del(&of->list);
 
-       if (atomic_dec_and_test(&on->refcnt))
+       if (list_empty(&on->files))
                kn->attr.open = NULL;
        else
                on = NULL;
@@ -768,15 +765,15 @@ void kernfs_drain_open_files(struct kernfs_node *kn)
        if (!(kn->flags & (KERNFS_HAS_MMAP | KERNFS_HAS_RELEASE)))
                return;
 
-       spin_lock_irq(&kernfs_open_node_lock);
        on = kn->attr.open;
-       if (on)
-               atomic_inc(&on->refcnt);
-       spin_unlock_irq(&kernfs_open_node_lock);
        if (!on)
                return;
 
        mutex_lock(&kernfs_open_file_mutex);
+       if (!kn->attr.open) {
+               mutex_unlock(&kernfs_open_file_mutex);
+               return;
+       }
 
        list_for_each_entry(of, &on->files, list) {
                struct inode *inode = file_inode(of->file);
@@ -789,8 +786,6 @@ void kernfs_drain_open_files(struct kernfs_node *kn)
        }
 
        mutex_unlock(&kernfs_open_file_mutex);
-
-       kernfs_put_open_node(kn, NULL);
 }
 
 /*