Re: [PATCH 1/3] rseq: optimize rseq_update_cpu_id()

[Eric Dumazet]

On Tue, Apr 13, 2021 at 4:29 PM Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>
> ----- On Apr 13, 2021, at 3:36 AM, Eric Dumazet eric.dumazet@xxxxxxxxx wrote:
>
> > From: Eric Dumazet <edumazet@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> > Cc: Arjun Roy <arjunroy@xxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > ---
> > 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.