linux-kernel - Re: [PATCH 1/2 v3] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED

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Message-ID: <CAFTs51XK0HLwCCvXCcfE5P7a4ExANPNPw7UvNigwHZ8sZVP+nQ@mail.gmail.com>
Date:   Tue, 11 Aug 2020 14:08:22 -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>,
        Boqun Feng <boqun.feng@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Chris Kennelly <ckennelly@...gle.com>
Subject: Re: [PATCH 1/2 v3] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ

On Mon, Aug 10, 2020 at 11:27 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Aug 10, 2020 at 05:09:58PM -0700, Peter Oskolkov wrote:
> > @@ -27,6 +35,12 @@
> >
> >  static void ipi_mb(void *info)
> >  {
>
> The #ifdef wants to behere, otherwise you'll get a compile warning for
> !RSEQ builds.

Ack. Will do in the next version - for now waiting for the rest to be
worked out.

[...]

>
> Mathieu did mention a few other points that I didn't see addressed:
>
>  - he didn't like abusing the @flags syscall argument for a CPUid;

@flags is not used now; maybe just rename it to something more
generic? @param? Or @options? Or maybe more specific, like @cpu_id?

>  - he wondered if we should support SYNC_CORE + RSEQ.

It seems to me that CMD_PRIVATE_EXPEDITED_RSEQ is basically
CMD_PRIVATE_EXPEDITED_SYNC_CORE with the extra "restart RSEQ CSs"
behavior. Am I missing something? If not, what is the point of
complicating the code as suggested below? Maybe just renaming
CMD_PRIVATE_EXPEDITED_RSEQ to CMD_PRIVATE_EXPEDITED_SYNC_CORE_RSEQ
will do?

>
>
> Not sure we can easily change the syscall at this point, but the latter
> point could be addressed with something like this.
>
> ---
> Index: linux-2.6/kernel/sched/membarrier.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched/membarrier.c
> +++ linux-2.6/kernel/sched/membarrier.c
> @@ -374,8 +374,26 @@ static int membarrier_register_private_e
>   */
>  SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
>  {
> +       int cflags = 0, int cpuid = -1;
> +
>         if (unlikely(flags) && cmd != MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ)
>                 return -EINVAL;
> +
> +       if (cmd & (MEMBARRIER_CMD_PRIVATE_EXPEDITED |
> +                  MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE |
> +                  MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ)) {
> +
> +               if (cmd & MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ)
> +                       cflags |= MEMBARRIER_FLAG_RSEQ;
> +
> +               if (cmd & MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE) {
> +                       cflags |= MEMBARRIER_FLAG_SYNC_CORE;
> +                       cpuid = flags;
> +               }
> +
> +               cmd = MEMBARRIER_CMD_PRIVATE_EXPEDITED;
> +       }
> +
>         switch (cmd) {
>         case MEMBARRIER_CMD_QUERY:
>         {
> @@ -396,18 +414,16 @@ SYSCALL_DEFINE2(membarrier, int, cmd, in
>                 return membarrier_global_expedited();
>         case MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED:
>                 return membarrier_register_global_expedited();
> -       case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
> -               return membarrier_private_expedited(0, -1);
>         case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED:
>                 return membarrier_register_private_expedited(0);
> -       case MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE:
> -               return membarrier_private_expedited(MEMBARRIER_FLAG_SYNC_CORE, -1);
>         case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE:
>                 return membarrier_register_private_expedited(MEMBARRIER_FLAG_SYNC_CORE);
> -       case MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ:
> -               return membarrier_private_expedited(MEMBARRIER_FLAG_RSEQ, flags);
>         case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ:
>                 return membarrier_register_private_expedited(MEMBARRIER_FLAG_RSEQ);
> +
> +       case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
> +               return membarrier_private_expedited(cflags, cpuid);
> +
>         default:
>                 return -EINVAL;
>         }
>