linux-kernel - Re: [patch 20/24] x86/speculation: Split out TIF update

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Message-ID: <nycvar.YFH.7.76.1811270820050.21108@cbobk.fhfr.pm>
Date:   Tue, 27 Nov 2018 08:30:07 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [patch 20/24] x86/speculation: Split out TIF update

On Tue, 27 Nov 2018, Thomas Gleixner wrote:

> That's racy and does not prevent the situation because the TIF flags are
> updated befor the UPDATE bit is set. 

> So __speculation_ctrl_update() might see the new bits, but not 
> TIF_SPEC_UPDATE. 

Hm, right, scratch that. We'd need to do that before updating TIF_SPEC_IB 
in task_update_spec_tif(), but that has the opposite ordering issue.

Thanks,

-- 
Jiri Kosina
SUSE Labs