Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751111AbdL3Upm (ORCPT <rfc822;w@1wt.eu>);
        Sat, 30 Dec 2017 15:45:42 -0500
Received: from mail-it0-f47.google.com ([209.85.214.47]:35961 "EHLO
        mail-it0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1750943AbdL3Upl (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Sat, 30 Dec 2017 15:45:41 -0500
X-Google-Smtp-Source: ACJfBougn79qE5XVsbsr1dx8qnGODY/OnQo9NjlA9Ly2njCILrCkRgjguGFkVLqFX3c8arKGwEy4RbJhBn/gThn2h5s=
MIME-Version: 1.0
In-Reply-To: <alpine.DEB.2.20.1712302121310.1899@nanos>
References: <20171230132927.GA2731@light.dominikbrodowski.net>
 <alpine.DEB.2.20.1712301558500.1899@nanos> <20171230153054.GA1604@light.dominikbrodowski.net>
 <alpine.DEB.2.20.1712301916020.1899@nanos> <CA+55aFybDbanqpA2eSy1oSCUj9a8RPuAgQYEZN8c+Rfege1a3Q@mail.gmail.com>
 <alpine.DEB.2.20.1712302121310.1899@nanos>
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sat, 30 Dec 2017 12:45:39 -0800
X-Google-Sender-Auth: qWpo52jCIcTcooosKjuXDxEYx88
Message-ID: <CA+55aFzfSWVXtswiS7iucQ8m+s8pbMqW_7H+RfEmdYxLtEAYcQ@mail.gmail.com>
Subject: Re: x86/pti: smp_processor_id() called while preemptible in resume-from-sleep
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 1032
Lines: 31

On Sat, Dec 30, 2017 at 12:28 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>
> That preempt_disable()/enable() was added with:
>
> commit 5cf0791da5c162ebc14b01eb01631cfa7ed4fa6e
> Author: Sebastian Andrzej Siewior <bigeasy@linutronix.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.

Good point, but I think that was actually a mis-feature of the old
"__flush_tlb_up()" implementation that Andy got rid of in commit
ce4a4e565f52 ("x86/mm: Remove the UP asm/tlbflush.h code, always use
the (formerly) SMP code").

So the code sequence that that commit talks about no longer exists.

Instead, we now have the call to __flush_tlb() inside a
get_cpu/put_cpu, which is preempt-safe even on UP (despite the CPU
number obviously being fixed).

So I think Dave is right: we should just remove the
preempt_disable/endable. But adding a

        WARN_ON_ONCE(preemptible());

might still be a good idea.

               Linus