linux-kernel - Re: [PATCH workqueue/for-6.10-fixes] workqueue: Refactor worker ID formatting and make wq_worker

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Date: Sun, 26 May 2024 10:34:08 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: bpf <bpf@...r.kernel.org>, Tejun Heo <tj@...nel.org>, Jan Engelhardt <jengelh@...i.de>, 
	Craig Small <csmall@....com.au>, linux-kernel@...r.kernel.org, 
	Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH workqueue/for-6.10-fixes] workqueue: Refactor worker ID
 formatting and make wq_worker_comm() use full ID string

On Fri, May 24, 2024 at 10:58 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Fri, 24 May 2024 at 00:43, Yafang Shao <laoar.shao@...il.com> wrote:
> >
> > Actually, there are already helpers for this: get_task_comm() and
> > __get_task_comm(). We can simply replace the memcpy() with one of
> > these
>
> No. We should get rid of those horrendous helpers.
>
> > If the task_lock() in __get_task_comm() is a concern, we could
> > consider adding a new __get_current_comm().
>
> The task_lock is indeed the problem - it generates locking problems
> and basically means that most places cannot use them. Certainly not
> things like tracing etc.
>
> The locking is also entirely pointless\, since absolutely nobody
> cares. If somebody is changing the name at the same time - which
> doesn't happen in practice - getting some halfway result is fine as
> long as you get a proper NUL terminated result.
>
> Even for non-current, they are largely useless. They were a mistake.
>
> So those functions should never be used for any normal thing. Instead
> of locking, the function should literally just do a "copy a couple of
> words and make sure the end result still has a NUL at the end".
>
> That's literally what selinuxfs.c wants, for example - it copies the
> thing to a local buffer not because it cares about some locking issue,
> but because it wants one stable value. But by using 'memcpy()' and
> that fixed size, it means that we can't sanely extend the source size
> because now it wouldn't be NUL-terminated. But selinux never wanted a
> lock, and never wanted any kind of *consistent* result, it just wanted
> a *stable* result.
>
> Since user space can randomly change their names anyway, using locking
> was always wrong for readers (for writers it probably does make sense
> to have some lock - although practically speaking nobody cares there
> either, but at least for a writer some kind of race could have
> long-term mixed results)

Thanks for your explanation.
Let's proceed by removing the task_lock() inside __get_task_comm().

-- 
Regards
Yafang