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Message-ID: <CALCETrWnw0WjionuGAT1F8r5y_t5CbYh+TmDL4s2d=ZbiinspA@mail.gmail.com>
Date: Mon, 11 Jul 2016 07:55:17 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Mark Rutland <mark.rutland@....com>
Cc: Nadav Amit <nadav.amit@...il.com>, linux-arch <linux-arch@...r.kernel.org>,
Kees Cook <keescook@...omium.org>, Josh Poimboeuf <jpoimboe@...hat.com>,
Borislav Petkov <bp@...en8.de>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Jann Horn <jann@...jh.net>,
Heiko Carstens <heiko.carstens@...ibm.com>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Brian Gerst <brgerst@...il.com>,
X86 ML <x86@...nel.org>, Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v4 26/29] sched: Allow putting
thread_info into task_struct
On Jul 11, 2016 3:08 AM, "Mark Rutland" <mark.rutland@....com> wrote:
>
> Hi,
>
> On Sun, Jun 26, 2016 at 02:55:48PM -0700, Andy Lutomirski wrote:
> > If an arch opts in by setting CONFIG_THREAD_INFO_IN_TASK_STRUCT,
> > then thread_info is defined as a single 'u32 flags' and is the first
> > entry of task_struct. thread_info::task is removed (it serves no
> > purpose if thread_info is embedded in task_struct), and
> > thread_info::cpu gets its own slot in task_struct.
> >
> > This is heavily based on a patch written by Linus.
>
> I've been considering how we'd implement this for arm64, and I suspect
> that we'll also need to fold our preempt_count into task_struct
> (following from the style of asm-generic/preempt.h).
>
> As far as I can see, we can't make our preempt-count a percpu variable
> as with x86, as our percpu ops themselves are based on disabling
> preemption.
How do you intend to find 'current' to get to the preempt count
without first disabling preemption?
>
> To that end, would it be possible to keep the thread_info definition per
> arch, even with CONFIG_THREAD_INFO_IN_TASK?
In principal, yes, but could you alternatively put it in
thread_struct? My goal here is to encourage people to clean up their
use of thread_info vs thread_struct at the same time. For x86, that
cleanup was trivial -- most of the work was addressing relative to
current instead of the stack pointer, and that had to happen
regardless.
--Andy
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