linux-kernel - Re: x86/pti: smp_processor_id() called while preemptible in resume-from-sleep

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Date:   Sat, 30 Dec 2017 21:28:03 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Dominik Brodowski <linux@...inikbrodowski.net>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: x86/pti: smp_processor_id() called while preemptible in
 resume-from-sleep

On Sat, 30 Dec 2017, Linus Torvalds wrote:
> On Sat, Dec 30, 2017 at 10:20 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> The __native_flush_tlb() function looks _very_ broken.
> 
> It does:
> 
>         invalidate_user_asid(this_cpu_read(cpu_tlbstate.loaded_mm_asid));
>         /*
>          * If current->mm == NULL then we borrow a mm which may change
>          * during a task switch and therefore we must not be preempted
>          * while we write CR3 back:
>          */
>         preempt_disable();
>         native_write_cr3(__native_read_cr3());
>         preempt_enable();
> 
> but why is that preempt-disabled region only around the cr3 write? The
> invalidate_user_asid() logic seems to be very CPU-sensitive too.
> 
> And even if there is some reason why invalidate_user_asid() really can
> do multiple different percpu accesses and it doesn't matter whether
> the thread is bouncing around on different cpu's while it does it,
> there doesn't seem any _reason_ not to just extend the preempt-disable
> over the whole series.
> 
> It really looks strange how it does multiple reads (and then a final
> write!) to percpu state, when the cpu can change in between.
> 
> So I'd suggest moving the preempt_disable() up to the top of that
> function,

That preempt_disable()/enable() was added with:

commit 5cf0791da5c162ebc14b01eb01631cfa7ed4fa6e
Author: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Date:   Fri Aug 5 15:37:39 2016 +0200

    x86/mm: Disable preemption during CR3 read+write

So we need to look at that scenario before removing it.

Thanks,

	tglx