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

[Jens Axboe <axboe@kernel.dk>]

On 4/14/25 11:11 AM, Mateusz Guzik wrote:
> 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>
>> ---
>>  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
> 
> I don't have comments on the semantics here, I do have comments on some
> future-proofing.
> 
> To my reading kthreads on the stock kernel never execute task_work.

Correct

> This suggests it would be nice for task_work_add() to at least WARN_ON
> when executing with a kthread. After all you don't want a task_work_add
> consumer adding work which will never execute.

I don't think there's much need for that, as I'm not aware of any kernel
usage that had a bug due to that. And if you did, you'd find it pretty
quick during testing as that work would just never execute.

> But then for your patch to not produce any splats there would have to be
> a flag blessing select kthreads as legitimate task_work consumers.

This patchset very much adds a specific flag for that, PF_NO_TASKWORK,
and kernel threads have it set by default. It just separates the "do I
run task_work" flag from PF_KTHREAD. So yes you could add:

WARN_ON_ONCE(task->flags & PF_NO_TASKWORK);

to task_work_add(), but I'm not really convinced it'd be super useful.

-- 
Jens Axboe