Re: [PATCH bpf-next v9 3/4] bpf: Add kfuncs for storing struct task_struct * as a kptr

[David Vernet]

On Mon, Dec 05, 2022 at 11:11:47AM +0100, Matus Jokay wrote:
> Hello David,

Hi Matus,

> 
> Your idea behind this patch is cool, but I'm afraid that the
> implementation is incorrect.
>
> As you can see, the task_struct:rcu_users member shares the same memory
> area with the task_struct:rcu (the head of an RCU CB).
> Consequence: *violated invariant* that the reference counter will
> remain zero after reaching zero!!!
> After reaching zero the task_struct:rcu head is set, so further attempts
> to access the task_struct:rcu_users may lead to a non-zero value.

Yes, you're right. Thanks for explaining this and pointing out the
oversight.

> For more information see
> https://lore.kernel.org/lkml/CAHk-=wjT6LG6sDaZtfeT80B9RaMP-y7RNRM4F5CX2v2Z=o8e=A@xxxxxxxxxxxxxx/
> In my opinion, the decision about task_struct:rcu and
> task_struct:rcu_users union is very bad, but you should probably consult
> the memory separation with authors of the actual implementation.

I expect the reason it's like that is because prior to this change, as
Linus pointed out, nothing ever increments the refcount (other than as
of commit 912616f142bf: ("exit: Guarantee make_task_dead leaks the tsk
when calling do_task_exit"), which similarly increments before the
reference could have ever gone to 0, so I think is fine), so we had the
ability to save a few bytes of memory in struct task_struct. Eric
mentioned this explicitly in the commit summary for commit 3fbd7ee285b2
("tasks: Add a count of task RCU users").

Now that the refcount is actually being used as a proper refcount with
this commit, that space saving is no longer an option (unless we rip out
my changes of course). +cc Eric and Oleg -- would you guys be OK with
separating them out from that union? I guess the alternative would be to
check for p->flags & PF_EXITING in the helper, but using p->rcu_users
feels more natural.

> For now, in our project, we use the following approach:
> 
> 1) get a reference to a valid task within RCU read-side with
>    get_task_struct()
> 2) in the release function:
>     2.1) enter RCU read-side
>     2.2) if the task state is not TASK_DEAD: use put_task_struct()
>          Note: In the case of a race with an exiting task it's OK to
>          call put_task_struct(), because task_struct will be freed
>          *after* the current RCU GP thanks to the task_struct:rcu_users
>          mechanism.
>     2.3) otherwise if test_and_set(my_cb_flag): call_rcu(my_cb)
>          Note1: With respect to the RCU CB API you should guarantee that
>          your CB will be installed only once within a given RCU GP. For
>          that purpose we use my_cb_flag.
>          Note2: This code will race with the task_struct:rcu_users
>          mechanism [delayed_put_task_struct()], but it's OK. Either the
>          delayed_put_task_struct() or my_cb() can be the last to call
>          final put_task_struct() after the current RCU GP.

I think this idea would work, but in order for us to do this, I believe
we'd have to add _another_ struct rcu_head to struct task_struct. If we
did that, I don't think there's any reason to not just separate them out
of the union where they live today as it's only like that for
space-saving reasons.

>     2.4) otherwise: call put_task_struct()
>          Note: The my_cb() is already installed, so within the current
>          RCU GP we can invoke put_task_struct() and the ref counter of
>          the task_struct will not reach zero.
>     2.5) release the RCU read-side
> 3) The RCU CB my_cb() should set the my_cb_flag to False and call
> put_task_struct().
> 
> If the release function is called within RCU read-side, the task_struct
> is guaranteed to remain valid until the end of the current RCU GP.
> 
> Good luck,
> mY