linux-kernel - Re: linux-next: manual merge of the rcu tree with the tip tree

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Message-ID: <CALCETrWWx8h=74td8bxbR6KZJN2GLAZ_jz9G-x9Q+Vn2AHSijQ@mail.gmail.com>
Date:   Tue, 1 Aug 2017 07:15:40 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: linux-next: manual merge of the rcu tree with the tip tree

On Tue, Aug 1, 2017 at 7:02 AM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> /*
>  * The full memory barrier implied by mm_cpumask update operations
>  * is required by the membarrier system call.
>  */
>
> What we want to order here is:
>
> prev userspace memory accesses
> schedule
>   <full mb> (it's already there) [A]
>   update to rq->curr changing the rq->curr->mm value
>   <full mb> (provided by mm_cpumask updates in switch_mm on x86) [B]

If I understand this right, the issue with relying on CR3 writes is
that the target CPU could switch to a kernel thread and back to the
same user mm white the membarrier caller is reading its mm, right?