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

[Mathieu Desnoyers]

----- 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.

Thanks,

Mathieu

> 
> -	if (put_user(cpu_id, &t->rseq->cpu_id_start))
> -		return -EFAULT;
> -	if (put_user(cpu_id, &t->rseq->cpu_id))
> -		return -EFAULT;
> +	if (!user_write_access_begin(r, sizeof(*r)))
> +		goto efault;
> +	unsafe_put_user(cpu_id, &r->cpu_id_start, efault_end);
> +	unsafe_put_user(cpu_id, &r->cpu_id, efault_end);
> +	user_write_access_end();
> 	trace_rseq_update(t);
> 	return 0;
> +
> +efault_end:
> +	user_write_access_end();
> +efault:
> +	return -EFAULT;
> }
> 
> static int rseq_reset_rseq_cpu_id(struct task_struct *t)
> --
> 2.31.1.295.g9ea45b61b8-goog

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com