linux-kernel - Re: [PATCH 1/2] membarrier: add MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON

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Date:   Thu, 6 Aug 2020 10:07:53 -0700
From:   Peter Oskolkov <posk@...k.io>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Peter Oskolkov <posk@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Chris Kennelly <ckennelly@...gle.com>
Subject: Re: [PATCH 1/2] membarrier: add MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU

On Thu, Aug 6, 2020 at 6:48 AM <peterz@...radead.org> wrote:
>
> On Wed, Aug 05, 2020 at 05:08:58PM -0700, Peter Oskolkov wrote:
>
> Thanks for the Cc!

Always a pleasure!

(Sorry, included only membarrier maintainers in v1; in v2 included
both membarrier and rseq maintainers).

>
> > + * @MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU:
> > + *                          If a thread belonging to the current process
> > + *                          is currently in an RSEQ critical section on the
> > + *                          CPU identified by flags parameter, restart it.
> > + *                          @flags: if @flags >= 0, identifies the CPU to
> > + *                                  restart RSEQ CS on; if == -1, restarts
> > + *                                  RSEQ CSs on all CPUs.
>
> > +     } else if (cpu_id == -1) {
> > +             on_each_cpu(membarrier_rseq_ipi,
> > +                         current->group_leader, true);
>
> This is an unpriv IPI the world. That's a big no-no.

removed in v2.

>
> Double so because all you want to target is the current process, which
> you're defining as CLONE_THREAD, where the rest of this file uses
> CLONE_VM to define a process.

Use current->mm in v2 instead of current->group_leader. Is it better this way?