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From:   Andi Kleen <ak@linux.intel.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Borislav Petkov <bp@suse.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>,
        Joerg Roedel <joro@8bytes.org>,
        Jun Nakajima <jun.nakajima@intel.com>
Subject: Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same user process
References: <c5cc429013c2248ddebe0a8480b172ae70b29733.1516840211.git.tim.c.chen@linux.intel.com>
        <20180125085820.GV2228@hirez.programming.kicks-ass.net>
        <20180125092233.GE2295@hirez.programming.kicks-ass.net>
        <86541aca-8de7-163d-b620-083dddf29184@linux.intel.com>
        <20180125135055.GK2249@hirez.programming.kicks-ass.net>
        <aeef4a08-7dd3-3d3a-02f4-9b8b4a1461bf@linux.intel.com>
        <20180125164139.GM2269@hirez.programming.kicks-ass.net>
        <CALCETrVeb7g+wAP8hox9TEBBWYAFXGzOv5t2aQVrtuW4D6NQQQ@mail.gmail.com>
        <20180125181852.GL2249@hirez.programming.kicks-ass.net>
        <aa7cfe53-d8ee-9f56-a5c7-775d878cee15@linux.intel.com>
        <20180125204625.GN2269@hirez.programming.kicks-ass.net>
        <CALCETrUGDeHse24sOygh5EcsXuZHPtZnw3adhzADfn+yKgcOzg@mail.gmail.com>
Date:   Thu, 25 Jan 2018 13:57:49 -0800
In-Reply-To: <CALCETrUGDeHse24sOygh5EcsXuZHPtZnw3adhzADfn+yKgcOzg@mail.gmail.com>
        (Andy Lutomirski's message of "Thu, 25 Jan 2018 13:04:23 -0800")
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Andy Lutomirski <luto@kernel.org> writes:
>
> That being said, just stashing last_user_mm without any refcounting
> should be fine.

If last_user_mm is freed and reallocated by a different process,
then that would miss the IPBP incorrectly.

-Andi