Re: [PATCH 1/5] fs: gate final fput task_work on PF_NO_TASKWORK

[Christian Brauner <brauner@kernel.org>]

On Wed, Apr 09, 2025 at 07:35:19AM -0600, Jens Axboe wrote:
> fput currently gates whether or not a task can run task_work on the
> PF_KTHREAD flag, which excludes kernel threads as they don't usually run
> task_work as they never exit to userspace. This punts the final fput
> done from a kthread to a delayed work item instead of using task_work.
> 
> It's perfectly viable to have the final fput done by the kthread itself,
> as long as it will actually run the task_work. Add a PF_NO_TASKWORK flag
> which is set by default by a kernel thread, and gate the task_work fput
> on that instead. This enables a kernel thread to clear this flag
> temporarily while putting files, as long as it runs its task_work
> manually.
> 
> This enables users like io_uring to ensure that when the final fput of a
> file is done as part of ring teardown to run the local task_work and
> hence know that all files have been properly put, without needing to
> resort to workqueue flushing tricks which can deadlock.
> 
> No functional changes in this patch.
> 
> Cc: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---

Seems fine. Although it has some potential for abuse. So maybe a
VFS_WARN_ON_ONCE() that PF_NO_TASKWORK is only used with PF_KTHREAD
would make sense.

Acked-by: Christian Brauner <brauner@kernel.org>

>  fs/file_table.c       | 2 +-
>  include/linux/sched.h | 2 +-
>  kernel/fork.c         | 2 +-
>  3 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/file_table.c b/fs/file_table.c
> index c04ed94cdc4b..e3c3dd1b820d 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -521,7 +521,7 @@ static void __fput_deferred(struct file *file)
>  		return;
>  	}
>  
> -	if (likely(!in_interrupt() && !(task->flags & PF_KTHREAD))) {
> +	if (likely(!in_interrupt() && !(task->flags & PF_NO_TASKWORK))) {
>  		init_task_work(&file->f_task_work, ____fput);
>  		if (!task_work_add(task, &file->f_task_work, TWA_RESUME))
>  			return;
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index f96ac1982893..349c993fc32b 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1736,7 +1736,7 @@ extern struct pid *cad_pid;
>  						 * I am cleaning dirty pages from some other bdi. */
>  #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
>  #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
> -#define PF__HOLE__00800000	0x00800000
> +#define PF_NO_TASKWORK		0x00800000	/* task doesn't run task_work */
>  #define PF__HOLE__01000000	0x01000000
>  #define PF__HOLE__02000000	0x02000000
>  #define PF_NO_SETAFFINITY	0x04000000	/* Userland is not allowed to meddle with cpus_mask */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index c4b26cd8998b..8dd0b8a5348d 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2261,7 +2261,7 @@ __latent_entropy struct task_struct *copy_process(
>  		goto fork_out;
>  	p->flags &= ~PF_KTHREAD;
>  	if (args->kthread)
> -		p->flags |= PF_KTHREAD;
> +		p->flags |= PF_KTHREAD | PF_NO_TASKWORK;
>  	if (args->user_worker) {
>  		/*
>  		 * Mark us a user worker, and block any signal that isn't
> -- 
> 2.49.0
>