linux-kernel - Re: [RFC][PATCH 6/7] context_tracking: Provide SMP ordering using RCU

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Date:   Fri, 24 Sep 2021 14:57:05 -0400
From:   Joel Savitz <jsavitz@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     gor@...ux.ibm.com, jpoimboe@...hat.com, jikos@...nel.org,
        mbenes@...e.cz, pmladek@...e.com, mingo@...nel.org,
        linux-kernel@...r.kernel.org, joe.lawrence@...hat.com,
        fweisbec@...il.com, tglx@...utronix.de, hca@...ux.ibm.com,
        svens@...ux.ibm.com, sumanthk@...ux.ibm.com,
        live-patching@...r.kernel.org, paulmck@...nel.org
Subject: Re: [RFC][PATCH 6/7] context_tracking: Provide SMP ordering using RCU

On Wed, Sep 22, 2021 at 01:05:12PM +0200, Peter Zijlstra wrote:
> ---
>  include/linux/context_tracking_state.h |   12 ++++++++++++
>  kernel/context_tracking.c              |    7 ++++---
>  2 files changed, 16 insertions(+), 3 deletions(-)
> 
> --- a/include/linux/context_tracking_state.h
> +++ b/include/linux/context_tracking_state.h
> @@ -45,11 +45,23 @@ static __always_inline bool context_trac
>  {
>  	return __this_cpu_read(context_tracking.state) == CONTEXT_USER;
>  }
> +
> +static __always_inline bool context_tracking_state_cpu(int cpu)
> +{
> +	struct context_tracking *ct = per_cpu_ptr(&context_tracking);
> +
> +	if (!context_tracking_enabled() || !ct->active)
> +		return CONTEXT_DISABLED;
> +
> +	return ct->state;
> +}
> +
>  #else
>  static inline bool context_tracking_in_user(void) { return false; }
>  static inline bool context_tracking_enabled(void) { return false; }
>  static inline bool context_tracking_enabled_cpu(int cpu) { return false; }
>  static inline bool context_tracking_enabled_this_cpu(void) { return false; }
> +static inline bool context_tracking_state_cpu(int cpu) { return CONTEXT_DISABLED; }
>  #endif /* CONFIG_CONTEXT_TRACKING */
>  
>  #endif

Should context_tracking_state_cpu return an enum ctx_state rather than a
bool? It appears to be doing an implicit cast.

I don't know if it possible to run livepatch with
CONFIG_CONTEXT_TRACKING disabled, but if so, then klp_check_task() as
modified by patch 7 will always consider the transition complete even if
the current task is in kernel mode. Also in the general case, the CPU
will consider the task complete if has ctx_state CONTEXT_GUEST though the
condition does not make it explicit.

I'm not sure what the correct behavior should be here as I am not very
experienced with this sybsystem but the patch looks a bit odd to me.

> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -82,7 +82,7 @@ void noinstr __context_tracking_enter(en
>  				vtime_user_enter(current);
>  				instrumentation_end();
>  			}
> -			rcu_user_enter();
> +			rcu_user_enter(); /* smp_mb */
>  		}
>  		/*
>  		 * Even if context tracking is disabled on this CPU, because it's outside
> @@ -149,12 +149,14 @@ void noinstr __context_tracking_exit(enu
>  		return;
>  
>  	if (__this_cpu_read(context_tracking.state) == state) {
> +		__this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
> +
>  		if (__this_cpu_read(context_tracking.active)) {
>  			/*
>  			 * We are going to run code that may use RCU. Inform
>  			 * RCU core about that (ie: we may need the tick again).
>  			 */
> -			rcu_user_exit();
> +			rcu_user_exit(); /* smp_mb */
>  			if (state == CONTEXT_USER) {
>  				instrumentation_begin();
>  				vtime_user_exit(current);
> @@ -162,7 +164,6 @@ void noinstr __context_tracking_exit(enu
>  				instrumentation_end();
>  			}
>  		}
> -		__this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
>  	}
>  	context_tracking_recursion_exit();
>  }
> 
>