Re: [PATCH 1/2] Allow a kthread to declare that it calls task_work_run()

[Jens Axboe <axboe@kernel.dk>]

On 12/5/23 2:58 PM, Jens Axboe wrote:
> On 12/5/23 2:28 PM, NeilBrown wrote:
>> On Tue, 05 Dec 2023, Christian Brauner wrote:
>>> On Mon, Dec 04, 2023 at 03:09:44PM -0700, Jens Axboe wrote:
>>>> On 12/4/23 2:02 PM, NeilBrown wrote:
>>>>> It isn't clear to me what _GPL is appropriate, but maybe the rules
>>>>> changed since last I looked..... are there rules?
>>>>>
>>>>> My reasoning was that the call is effectively part of the user-space
>>>>> ABI.  A user-space process can call this trivially by invoking any
>>>>> system call.  The user-space ABI is explicitly a boundary which the GPL
>>>>> does not cross.  So it doesn't seem appropriate to prevent non-GPL
>>>>> kernel code from doing something that non-GPL user-space code can
>>>>> trivially do.
>>>>
>>>> By that reasoning, basically everything in the kernel should be non-GPL
>>>> marked. And while task_work can get used by the application, it happens
>>>> only indirectly or implicitly. So I don't think this reasoning is sound
>>>> at all, it's not an exported ABI or API by itself.
>>>>
>>>> For me, the more core of an export it is, the stronger the reason it
>>>> should be GPL. FWIW, I don't think exporting task_work functionality is
> 
>>>
>>> Yeah, I'm not too fond of that part as well. I don't think we want to
>>> give modules the ability to mess with task work. This is just asking for
>>> trouble.
>>>
>>
>> Ok, maybe we need to reframe the problem then.
>>
>> Currently fput(), and hence filp_close(), take control away from kernel
>> threads in that they cannot be sure that a "close" has actually
>> completed.
>>
>> This is already a problem for nfsd.  When renaming a file, nfsd needs to
>> ensure any cached "open" that it has on the file is closed (else when
>> re-exporting an NFS filesystem it can result in a silly-rename).
>>
>> nfsd currently handles this case by calling flush_delayed_fput().  I
>> suspect you are no more happy about exporting that than you are about
>> exporting task_work_run(), but this solution isn't actually 100%
>> reliable.  If some other thread calls flush_delayed_fput() between nfsd
>> calling filp_close() and that same nfsd calling flush_delayed_fput(),
>> then the second flush can return before the first flush (in the other
>> thread) completes all the work it took on.
>>
>> What we really need - both for handling renames and for avoiding
>> possible memory exhaustion - is for nfsd to be able to reliably wait for
>> any fput() that it initiated to complete.
>>
>> How would you like the VFS to provide that service?
> 
> Since task_work happens in the context of your task already, why not
> just have a way to get it stashed into a list when final fput is done?
> This avoids all of this "let's expose task_work" and using the task list
> for that, which seems kind of pointless as you're just going to run it
> later on manually anyway.
> 
> In semi pseudo code:
> 
> bool fput_put_ref(struct file *file)
> {
> 	return atomic_dec_and_test(&file->f_count);
> }
> 
> void fput(struct file *file)
> {
> 	if (fput_put_ref(file)) {
> 		...
> 	}
> }
> 
> and then your nfsd_file_free() could do:
> 
> ret = filp_flush(file, id);
> if (fput_put_ref(file))
> 	llist_add(&file->f_llist, &l->to_free_llist);
> 
> or something like that, where l->to_free_llist is where ever you'd
> otherwise punt the actual freeing to.

Should probably have the put_ref or whatever helper also init the
task_work, and then reuse the list in the callback_head there. Then
whoever flushes it has to call ->func() and avoid exposing ____fput() to
random users. But you get the idea.

-- 
Jens Axboe