fput: turn "list_head delayed_fput_list" into llist_head

fput() and delayed_fput() can use llist and avoid the locking.

This is unlikely path, it is not that this change can improve
the performance, but this way the code looks simpler.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrey Vagin <avagin@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/file_table.c

@@ -265,18 +265,15 @@ static void __fput(struct file *file)
 	mntput(mnt);
 }
 
-static DEFINE_SPINLOCK(delayed_fput_lock);
-static LIST_HEAD(delayed_fput_list);
+static LLIST_HEAD(delayed_fput_list);
 static void delayed_fput(struct work_struct *unused)
 {
-	LIST_HEAD(head);
-	spin_lock_irq(&delayed_fput_lock);
-	list_splice_init(&delayed_fput_list, &head);
-	spin_unlock_irq(&delayed_fput_lock);
-	while (!list_empty(&head)) {
-		struct file *f = list_first_entry(&head, struct file, f_u.fu_list);
-		list_del_init(&f->f_u.fu_list);
-		__fput(f);
+	struct llist_node *node = llist_del_all(&delayed_fput_list);
+	struct llist_node *next;
+
+	for (; node; node = next) {
+		next = llist_next(node);
+		__fput(llist_entry(node, struct file, f_u.fu_llist));
 	}
 }
 
@@ -306,7 +303,6 @@ void fput(struct file *file)
 {
 	if (atomic_long_dec_and_test(&file->f_count)) {
 		struct task_struct *task = current;
-		unsigned long flags;
 
 		file_sb_list_del(file);
 		if (likely(!in_interrupt() && !(task->flags & PF_KTHREAD))) {
@@ -320,10 +316,9 @@ void fput(struct file *file)
 			 * fput to avoid leaking *file.
 			 */
 		}
-		spin_lock_irqsave(&delayed_fput_lock, flags);
-		list_add(&file->f_u.fu_list, &delayed_fput_list);
-		schedule_work(&delayed_fput_work);
-		spin_unlock_irqrestore(&delayed_fput_lock, flags);
+
+		if (llist_add(&file->f_u.fu_llist, &delayed_fput_list))
+			schedule_work(&delayed_fput_work);
 	}
 }
 

include/linux/fs.h

@@ -10,6 +10,7 @@
 #include <linux/stat.h>
 #include <linux/cache.h>
 #include <linux/list.h>
+#include <linux/llist.h>
 #include <linux/radix-tree.h>
 #include <linux/rbtree.h>
 #include <linux/init.h>
@@ -768,6 +769,7 @@ struct file {
 	 */
 	union {
 		struct list_head	fu_list;
+		struct llist_node	fu_llist;
 		struct rcu_head 	fu_rcuhead;
 	} f_u;
 	struct path		f_path;