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

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Message-ID: <CAFTs51WfWCGPDXy7YVNmfE8zjk98L8+V4erWUTofeYOJBJN-WQ@mail.gmail.com>
Date:   Tue, 25 Aug 2020 10:22:38 -0700
From:   Peter Oskolkov <posk@...k.io>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     linux-arch <linux-arch@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Peter Oskolkov <posk@...gle.com>, paulmck <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel <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 Tue, Aug 25, 2020 at 9:58 AM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
[...]
>
> Concretely speaking, let's just add a new membarrier command for the use-case
> at hand. All other ways of doing things we have discussed are tricky to expose
> in a way that is discoverable by user-space through the QUERY command. (using
> a flag, or OR'ing many commands together)
>
> >
> > 2: should @flags be repurposed for cpu_id, or MEMBARRIER_FLAG_CPU
> >   added with a new syscall parameter.
> > => I'm still not sure a new parameter can be cleanly added, but I can try
> >   it in the next patchset if you prefer it this way.
>
> Yes please, it's easy to implement and we'll quickly see if anyone yells. If
> it turns out to be a bad idea, you can always blame me. ;-)
>
> In summary:
>
> - We add 2 new membarrier commands:
>   - MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ
>   - MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ
>
> - We reserve a membarrier flag:
>
> enum membarrier_flag {
>   MEMBARRIER_FLAG_CPU = (1 << 0),
> }
>
> So for CMD_PRIVATE_EXPEDITED_RSEQ, if flags & MEMBARRIER_FLAG_CPU is true,
> then we expect the additional "int cpu" parameter (3rd parameter). Else the cpu
> parameter is unused.
>
> Are you OK with this approach ?

Yes, thanks for looking into this. I'll send a v4 later this week.

Thanks,
Peter

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