switch fput to task_work_add

... and schedule_work() for interrupt/kernel_thread callers
(and yes, now it *is* OK to call from interrupt).

We are guaranteed that __fput() will be done before we return
to userland (or exit).  Note that for fput() from a kernel
thread we get an async behaviour; it's almost always OK, but
sometimes you might need to have __fput() completed before
you do anything else.  There are two mechanisms for that -
a general barrier (flush_delayed_fput()) and explicit
__fput_sync().  Both should be used with care (as was the
case for fput() from kernel threads all along).  See comments
in fs/file_table.c for details.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/file_table.c

@@ -23,6 +23,8 @@
#include <linux/lglock.h>
#include <linux/percpu_counter.h>
#include <linux/percpu.h>
#include <linux/hardirq.h>
#include <linux/task_work.h>
#include <linux/ima.h>

#include <linux/atomic.h>
@@ -251,7 +253,6 @@ static void __fput(struct file *file)
	}
	fops_put(file->f_op);
	put_pid(file->f_owner.pid);
	file_sb_list_del(file);
	if ((file->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
		i_readcount_dec(inode);
	if (file->f_mode & FMODE_WRITE)
@@ -263,10 +264,77 @@ static void __fput(struct file *file)
	mntput(mnt);
}

static DEFINE_SPINLOCK(delayed_fput_lock);
static LIST_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);
	}
}

static void ____fput(struct callback_head *work)
{
	__fput(container_of(work, struct file, f_u.fu_rcuhead));
}

/*
 * If kernel thread really needs to have the final fput() it has done
 * to complete, call this.  The only user right now is the boot - we
 * *do* need to make sure our writes to binaries on initramfs has
 * not left us with opened struct file waiting for __fput() - execve()
 * won't work without that.  Please, don't add more callers without
 * very good reasons; in particular, never call that with locks
 * held and never call that from a thread that might need to do
 * some work on any kind of umount.
 */
void flush_delayed_fput(void)
{
	delayed_fput(NULL);
}

static DECLARE_WORK(delayed_fput_work, delayed_fput);

void fput(struct file *file)
{
	if (atomic_long_dec_and_test(&file->f_count))
	if (atomic_long_dec_and_test(&file->f_count)) {
		struct task_struct *task = current;
		file_sb_list_del(file);
		if (unlikely(in_interrupt() || task->flags & PF_KTHREAD)) {
			unsigned long flags;
			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);
			return;
		}
		init_task_work(&file->f_u.fu_rcuhead, ____fput);
		task_work_add(task, &file->f_u.fu_rcuhead, true);
	}
}

/*
 * synchronous analog of fput(); for kernel threads that might be needed
 * in some umount() (and thus can't use flush_delayed_fput() without
 * risking deadlocks), need to wait for completion of __fput() and know
 * for this specific struct file it won't involve anything that would
 * need them.  Use only if you really need it - at the very least,
 * don't blindly convert fput() by kernel thread to that.
 */
void __fput_sync(struct file *file)
{
	if (atomic_long_dec_and_test(&file->f_count)) {
		struct task_struct *task = current;
		file_sb_list_del(file);
		BUG_ON(!(task->flags & PF_KTHREAD));
		__fput(file);
	}
}

EXPORT_SYMBOL(fput);

include/linux/file.h

@@ -39,4 +39,7 @@ extern void put_unused_fd(unsigned int fd);

extern void fd_install(unsigned int fd, struct file *file);

extern void flush_delayed_fput(void);
extern void __fput_sync(struct file *);

#endif /* __LINUX_FILE_H */

init/main.c

@@ -68,6 +68,7 @@
#include <linux/shmem_fs.h>
#include <linux/slab.h>
#include <linux/perf_event.h>
#include <linux/file.h>

#include <asm/io.h>
#include <asm/bugs.h>
@@ -804,8 +805,8 @@ static noinline int init_post(void)
	system_state = SYSTEM_RUNNING;
	numa_default_policy();


	current->signal->flags |= SIGNAL_UNKILLABLE;
	flush_delayed_fput();

	if (ramdisk_execute_command) {
		run_init_process(ramdisk_execute_command);