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

Joel Savitz
2021-09-24T18:57:11Z

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();
> }
>
>