linux-kernel - Re: rust: wrong SAFETY comments in group

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Message-ID: <aTocSJ_zlTkejD5F@google.com>
Date: Thu, 11 Dec 2025 01:20:08 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Christian Brauner <christian@...uner.io>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, Panagiotis Foliadis <pfoliadis@...teo.net>, 
	Shankari Anand <shankari.ak0208@...il.com>, FUJITA Tomonori <fujita.tomonori@...il.com>, 
	Alexey Gladkov <legion@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: rust: wrong SAFETY comments in group_leader() and pid() + questions

On Mon, Dec 08, 2025 at 03:30:00PM +0100, Oleg Nesterov wrote:
> Alice,
> 
> Thanks again for your explanations. Not that I fully understand
> them, though ;)
> 
> On 12/05, Alice Ryhl wrote:
> >
> > To start with, it's likely that this comment is not the right choice
> > for this function, given our discussion. Most likely group_leader()
> > needs to be moved to `impl CurrentTask {}`
> 
> I obviously can't comment this proposal,
> 
> > and the safety comment needs
> > to explain why being the current task ensures that the returned &Task
> > lives for long enough.
> 
> This is simple. task->group_leader can't change or go away until
> this task exits or execs. The "current" task can't exit/exec.
> 
> (This also covers the race with mt-exec from current's subthread,
>  the execing thread will kill all the threads and wait until they
>  all pass release_task(). Only then it will change ->group_leader).
> 
> > impl CurrentTask {
> >     fn group_leader(&self) -> &Task {
> >         // SAFETY: This is the current task, so the task must be alive.
> >         // Therefore the group leader cannot change, and thus it will
> >         // stay valid as long as self is the current task.
> >         unsafe { &*bindings::task_group_leader(self.as_ptr()).cast::<Task>() }
> >     }
> > }
> 
> Yes, the comment looks good to me.
> 
> But we don't have the task_group_leader() helper yet, so far I
> only sent the trivial initial preparations, see
> https://lore.kernel.org/all/aTV1KYdcDGvjXHos@redhat.com/
> 
> So if you are going to move Task::group_leader to the
> CurrentTask block, please use .group_leader directly, like
> the current code does.

Yeah I'll be moving it soon (after conferences). I'll add a Reported-by
tag for you if that's okay :)

Alice