Re: [RFC][PATCH 6/7] context_tracking: Provide SMP ordering using RCU
From: Joel Savitz <hidden>
Date: 2021-09-24 18:57:12
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lkml
On Wed, Sep 22, 2021 at 01:05:12PM +0200, Peter Zijlstra wrote:
quoted hunk ↗ jump to hunk
--- 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.
quoted hunk ↗ jump to hunk
--- 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(); }