linux-kernel - Re: [PATCH 16/19] context_tracking: Convert state to atomic

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Message-ID: <23cbcd6a467c211f792ff54d50635cd6536c9dcc.camel@kernel.org>
Date:   Wed, 09 Mar 2022 18:17:02 +0100
From:   nicolas saenz julienne <nsaenz@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Phil Auld <pauld@...hat.com>,
        Alex Belits <abelits@...vell.com>,
        Xiongfeng Wang <wangxiongfeng2@...wei.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yu Liao <liaoyu15@...wei.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Uladzislau Rezki <uladzislau.rezki@...y.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 16/19] context_tracking: Convert state to atomic_t

On Wed, 2022-03-02 at 16:48 +0100, Frederic Weisbecker wrote:
> Context tracking's state and dynticks counter are going to be merged
> in a single field so that both updates can happen atomically and at the
> same time. Prepare for that with converting the state into an atomic_t.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
> Cc: Uladzislau Rezki <uladzislau.rezki@...y.com>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Nicolas Saenz Julienne <nsaenz@...nel.org>
> Cc: Marcelo Tosatti <mtosatti@...hat.com>
> Cc: Xiongfeng Wang <wangxiongfeng2@...wei.com>
> Cc: Yu Liao<liaoyu15@...wei.com>
> Cc: Phil Auld <pauld@...hat.com>
> Cc: Paul Gortmaker<paul.gortmaker@...driver.com>
> Cc: Alex Belits <abelits@...vell.com>
> ---
>  static __always_inline bool context_tracking_in_user(void)
>  {
> -	return __this_cpu_read(context_tracking.state) == CONTEXT_USER;
> +	return __ct_state() == CONTEXT_USER;
>  }

I was wondering whether it'd make more sense to use ct_state() for extra safety
vs preemption, but it turns out the function isn't being used at all.

I figure it'd be better to remove it altogether and leave ct_state() as the
goto function for this sort of checks.

>  #else
>  static inline bool context_tracking_in_user(void) { return false; }
> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> index de247e758767..69db43548768 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -337,6 +337,7 @@ static __always_inline void context_tracking_recursion_exit(void)
>   */
>  void noinstr __ct_user_enter(enum ctx_state state)
>  {
> +	struct context_tracking *ct = this_cpu_ptr(&context_tracking);

I wonder if there is any value to having __ct_state() take 'struct
context_tracking *ct' as an argument to avoid a redundant this_cpu_ptr()...

>  	lockdep_assert_irqs_disabled();
>  
>  	/* Kernel threads aren't supposed to go to userspace */
> @@ -345,8 +346,8 @@ void noinstr __ct_user_enter(enum ctx_state state)
>  	if (!context_tracking_recursion_enter())
>  		return;
>  
> -	if ( __this_cpu_read(context_tracking.state) != state) {
> -		if (__this_cpu_read(context_tracking.active)) {
> +	if (__ct_state() != state) {

...here (and in __ct_user_exit()).

Regards,
Nicolas