linux-kernel - Re: [PATCH 1/1] sched.h: always_inline alloc_tag

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Message-ID: <CAJuCfpEO3Lo9EZGQtdPUeyycbNwqJ7zvHq2N9nzd_Cc6pARchw@mail.gmail.com>
Date: Wed, 3 Jul 2024 21:25:30 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, 
	vincent.guittot@...aro.org, kent.overstreet@...ux.dev, 
	linux-kernel@...r.kernel.org, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 1/1] sched.h: always_inline alloc_tag_{save|restore} to
 fix modpost warnings

On Wed, Jul 3, 2024 at 9:17 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Wed, 3 Jul 2024 21:07:56 -0700 Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> > On Wed, Jul 3, 2024 at 8:54 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> > >
> > > On Wed, 3 Jul 2024 20:46:11 -0700 Suren Baghdasaryan <surenb@...gle.com> wrote:
> > >
> > > > Ok, I confirmed that the warning is happening due to the access to
> > > > "current" from alloc_tag_save()/alloc_tag_restore() functions. I guess
> > > > when these functions access "thread_info" variable:
> > > > https://elixir.bootlin.com/linux/v6.10-rc6/source/arch/xtensa/include/asm/thread_info.h#L96,
> > > > compiler flags that because the variable is on the stack of an __init
> > > > function while alloc_tag_save()/alloc_tag_restore() when not inlined
> > > > are from .text section.
> > >
> > > Well, is the warning legitimate?  I don't see why an automatic variable
> > > of an __init function should be considered to be .init storage - we can
> > > assume it won't become an invalid reference while the .init function is
> > > executing?
> >
> > I don't think it's really a problem. __init function is executing, it
> > calls a function from .text (say alloc_tag_save() that was not
> > inlined) which in turn calls get_current(), which returns a pointer
> > somewhere inside __initdata. That should be fine since this can only
> > happen during init stage. If this call happens after init,
> > get_current() can't return a pointer from __initdata. If it does then
> > we have a much bigger problem.
>
> So I think you're saying "yes, the warning is legitimate and this might
> be a problem, but it isn't in this case, so the checking code is OK so
> let's just work around it"?

Yes, that's my understanding. If get_current() always returns a valid
pointer for the given execution stage (during init/after init) then it
should be safe to call it even from a function residing in the .text
section during init.

>