linux-kernel - Re: [PATCH 1/3] rseq: optimize rseq_update_cpu

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Message-ID: <CANn89iJ28npWmB5F4haTz-PxV0=Vxm7DV9bkEkVJ=aPn=bVT0A@mail.gmail.com>
Date:   Tue, 13 Apr 2021 17:24:37 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        Arjun Roy <arjunroy@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] rseq: optimize rseq_update_cpu_id()

On Tue, Apr 13, 2021 at 4:29 PM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> ----- On Apr 13, 2021, at 3:36 AM, Eric Dumazet eric.dumazet@...il.com wrote:
>
> > From: Eric Dumazet <edumazet@...gle.com>
> >
> > Two put_user() in rseq_update_cpu_id() are replaced
> > by a pair of unsafe_put_user() with appropriate surroundings.
> >
> > This removes one stac/clac pair on x86 in fast path.
> >
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: "Paul E. McKenney" <paulmck@...nel.org>
> > Cc: Boqun Feng <boqun.feng@...il.com>
> > Cc: Arjun Roy <arjunroy@...gle.com>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > ---
> > kernel/rseq.c | 15 +++++++++++----
> > 1 file changed, 11 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/rseq.c b/kernel/rseq.c
> > index
> > a4f86a9d6937cdfa2f13d1dcc9be863c1943d06f..d2689ccbb132c0fc8ec0924008771e5ee1ca855e
> > 100644
> > --- a/kernel/rseq.c
> > +++ b/kernel/rseq.c
> > @@ -84,13 +84,20 @@
> > static int rseq_update_cpu_id(struct task_struct *t)
> > {
> >       u32 cpu_id = raw_smp_processor_id();
> > +     struct rseq *r = t->rseq;
>
> AFAIU the variable above should be a struct rseq __user *.
>
> Elsewhere in the file we use "rseq" rather than "r" for struct rseq __user *
> variable name, it would be better to keep the naming consistent across the file
> if possible.

Absolutely, thanks for the feedback.