linux-kernel - Re: [PATCH 2/5] context_tracking: Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING

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Date:   Wed, 11 Nov 2020 15:32:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Phil Auld <pauld@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH 2/5] context_tracking:  Don't implement
 exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK

On Tue, Oct 27, 2020 at 04:08:24PM +0100, Frederic Weisbecker wrote:
> An architecture that provides this Kconfig feature doesn't need to
> store the context tracking state on the task stack because its entry
> code has been sanitized such that fragile path aren't preemptible
> and special use of tracing and RCU read side critical sections in these
> areas have been explicitly annotated.
> 
> Hence the exception_enter()/exception_exit() couple doesn't need to be
> implemented in this case.

Could you please explain what exception_{enter,exit}() actually do, then
explain what is required to make it superfluous? Because as is, I don't
have enough information to verify the claims made.

> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Cc: Marcelo Tosatti <mtosatti@...hat.com>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Phil Auld <pauld@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> ---
>  include/linux/context_tracking.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
> index d53cd331c4dd..bceb06498521 100644
> --- a/include/linux/context_tracking.h
> +++ b/include/linux/context_tracking.h
> @@ -51,7 +51,8 @@ static inline enum ctx_state exception_enter(void)
>  {
>  	enum ctx_state prev_ctx;
>  
> -	if (!context_tracking_enabled())
> +	if (IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) ||
> +	    !context_tracking_enabled())
>  		return 0;
>  
>  	prev_ctx = this_cpu_read(context_tracking.state);
> @@ -63,7 +64,8 @@ static inline enum ctx_state exception_enter(void)
>  
>  static inline void exception_exit(enum ctx_state prev_ctx)
>  {
> -	if (context_tracking_enabled()) {
> +	if (!IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) &&
> +	    context_tracking_enabled()) {
>  		if (prev_ctx != CONTEXT_KERNEL)
>  			context_tracking_enter(prev_ctx);
>  	}
> -- 
> 2.25.1
>