linux-kernel - Re: [PATCH v5 2/5] pidfd: add PIDFD_SELF_* sentinels to refer to own thread/process

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Message-ID: <e5ac648b-88d7-4fa6-8eb4-d061a4b2baac@nvidia.com>
Date: Fri, 25 Oct 2024 11:44:34 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Pedro Falcato <pedro.falcato@...il.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Christian Brauner <christian@...uner.io>, Shuah Khan <shuah@...nel.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Suren Baghdasaryan <surenb@...gle.com>, Vlastimil Babka <vbabka@...e.cz>,
 linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
 linux-kernel@...r.kernel.org, Oliver Sang <oliver.sang@...el.com>
Subject: Re: [PATCH v5 2/5] pidfd: add PIDFD_SELF_* sentinels to refer to own
 thread/process

On 10/25/24 11:38 AM, Pedro Falcato wrote:
> On Fri, Oct 25, 2024 at 6:41 PM John Hubbard <jhubbard@...dia.com> wrote:
>>
>> On 10/25/24 5:50 AM, Pedro Falcato wrote:
>>> On Fri, Oct 25, 2024 at 10:41 AM Lorenzo Stoakes
>>> <lorenzo.stoakes@...cle.com> wrote:
>> ...
>>>> +static inline int pidfd_is_self_sentinel(pid_t pid)
>>>> +{
>>>> +       return pid == PIDFD_SELF_THREAD || pid == PIDFD_SELF_THREAD_GROUP;
>>>> +}
>>>
>>> Do we want this in the uapi header? Even if this is useful, it might
>>> come with several drawbacks such as breaking scripts that parse kernel
>>> headers (and a quick git grep suggests we do have static inlines in
>>> headers, but in rather obscure ones) and breaking C89:
>>>
>>
>> Let's please not say "C89" anymore, we've moved on! :)
>>
>> The notes in [1], which is now nearly 2.5 years old, discuss the move to
>> C11, and specifically how to handle the inline keyword.
> 
> That seems to only apply to the kernel internally, uapi headers are

Yes.

> included from userspace too (-std=c89 -pedantic doesn't know what a
> gnu extension is). And uapi headers _generally_ keep to defining
> constants and structs, nothing more.

OK

> I don't know what the guidelines for uapi headers are nowadays, but we
> generally want to not break userspace.
> 
>>
>> I think it's quite clear at this point, that we should not hold up new
>> work, based on concerns about handling the inline keyword, nor about
>> C89.
> 
> Right, but the correct solution is probably to move
> pidfd_is_self_sentinel to some other place, since it's not even
> supposed to be used by userspace (it's semantically useless to
> userspace, and it's only two users are in the kernel, kernel/pid.c and
> exit.c).
> 

Yes, if userspace absolutely doesn't need nor want this, then putting
it in a non-uapi header does sound like the right move.

thanks,
-- 
John Hubbard