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

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Message-Id: <6CCF28F9-CC33-412A-AC3C-208A36600D37@amacapital.net>
Date:   Sat, 30 Dec 2017 11:32:13 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Andy Lutomirski <luto@...nel.org>,
        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



--Andy

> On Dec 30, 2017, at 11:15 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> On Sat, Dec 30, 2017 at 11:03 AM, Dave Hansen
> <dave.hansen@...ux.intel.com> wrote:
>> On 12/30/2017 10:40 AM, Linus Torvalds wrote:
>>> The __native_flush_tlb() function looks _very_ broken.
>> ...
>>> So I'd suggest moving the preempt_disable() up to the top of that
>>> function, regardless of whether we could then remove that seemingly
>>> stale TLB flush in that crazy
>>> smpboot_setup/restore_warm_reset_vector() dance...
>> 
>> If someone is calling __native_flush_tlb(), shouldn't they already be in
>> a state where they can't be preempted?  It's fundamentally a one-cpu
>> thing, both the actual CPU TLB flush _and_ the per-cpu variables.
> 
> Hmm. I think you're right.
> 
>> It seems like we might want to _remove_ the explicit
>> preempt_dis/enable() from here:
>> 
>>        preempt_disable();
>>        native_write_cr3(__native_read_cr3());
>>        preempt_enable();
>> 
>> and add some warnings to ensure it's disabled when we enter
>> __native_flush_tlb().
> 
> Agreed, that would certainly also be consistent.
> 
> The current code that disables preemption only selectively seems
> insane to me. Either all or nothing, not this crazy half-way thing.

Agreed.  The current code is bogus.  I'd rather have a warning if preemptible.

I'm reasonably confident that IRQs on but preempt off is okay.

> 
>            Linus