Re: [PATCH v1 5/8] tracing: Allow system call tracepoints to handle page faults

[Steven Rostedt]

On Thu,  3 Oct 2024 11:16:35 -0400
Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:

> Use Tasks Trace RCU to protect iteration of system call enter/exit
> tracepoint probes to allow those probes to handle page faults.
> 
> In preparation for this change, all tracers registering to system call
> enter/exit tracepoints should expect those to be called with preemption
> enabled.
> 
> This allows tracers to fault-in userspace system call arguments such as
> path strings within their probe callbacks.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Michael Jeanson <mjeanson@xxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Yonghong Song <yhs@xxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> Cc: bpf@xxxxxxxxxxxxxxx
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> ---
>  include/linux/tracepoint.h | 25 +++++++++++++++++--------
>  init/Kconfig               |  1 +
>  2 files changed, 18 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index 666499b9f3be..6faf34e5efc9 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -17,6 +17,7 @@
>  #include <linux/errno.h>
>  #include <linux/types.h>
>  #include <linux/rcupdate.h>
> +#include <linux/rcupdate_trace.h>
>  #include <linux/tracepoint-defs.h>
>  #include <linux/static_call.h>
>  
> @@ -109,6 +110,7 @@ void for_each_tracepoint_in_module(struct module *mod,
>  #ifdef CONFIG_TRACEPOINTS
>  static inline void tracepoint_synchronize_unregister(void)
>  {
> +	synchronize_rcu_tasks_trace();
>  	synchronize_srcu(&tracepoint_srcu);
>  	synchronize_rcu();
>  }
> @@ -211,7 +213,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>   * it_func[0] is never NULL because there is at least one element in the array
>   * when the array itself is non NULL.
>   */
> -#define __DO_TRACE(name, args, cond, rcuidle)				\
> +#define __DO_TRACE(name, args, cond, rcuidle, syscall)			\
>  	do {								\
>  		int __maybe_unused __idx = 0;				\
>  									\
> @@ -222,8 +224,12 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  			      "Bad RCU usage for tracepoint"))		\
>  			return;						\
>  									\
> -		/* keep srcu and sched-rcu usage consistent */		\
> -		preempt_disable_notrace();				\
> +		if (syscall) {						\
> +			rcu_read_lock_trace();				\
> +		} else {						\
> +			/* keep srcu and sched-rcu usage consistent */	\
> +			preempt_disable_notrace();			\
> +		}							\
>  									\

I'm thinking we just use rcu_read_lock_trace() and get rid of the
preempt_disable and srcu locks for all tracepoints. Oh crap! I should get
rid of srcu locking too, as it was only needed for the rcuidle code :-p

-- Steve


>  		/*							\
>  		 * For rcuidle callers, use srcu since sched-rcu	\
> @@ -241,7 +247,10 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  			srcu_read_unlock_notrace(&tracepoint_srcu, __idx);\
>  		}							\
>  									\
> -		preempt_enable_notrace();				\
> +		if (syscall)						\
> +			rcu_read_unlock_trace();			\
> +		else							\
> +			preempt_enable_notrace();			\
>  	} while (0)
>  
>  #ifndef MODULE
> @@ -251,7 +260,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  		if (static_key_false(&__tracepoint_##name.key))		\
>  			__DO_TRACE(name,				\
>  				TP_ARGS(args),				\
> -				TP_CONDITION(cond), 1);			\
> +				TP_CONDITION(cond), 1, 0);		\
>  	}
>  #else
>  #define __DECLARE_TRACE_RCU(name, proto, args, cond)
> @@ -284,7 +293,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  		if (static_key_false(&__tracepoint_##name.key))		\
>  			__DO_TRACE(name,				\
>  				TP_ARGS(args),				\
> -				TP_CONDITION(cond), 0);			\
> +				TP_CONDITION(cond), 0, 0);		\
>  		if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) {		\
>  			WARN_ONCE(!rcu_is_watching(),			\
>  				  "RCU not watching for tracepoint");	\
> @@ -295,7 +304,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  		if (static_key_false(&__tracepoint_##name.key))		\
>  			__DO_TRACE(name,				\
>  				TP_ARGS(args),				\
> -				TP_CONDITION(cond), 1);			\
> +				TP_CONDITION(cond), 1, 0);		\
>  	}								\
>  	static inline int						\
>  	register_trace_##name(void (*probe)(data_proto), void *data)	\
> @@ -330,7 +339,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  		if (static_key_false(&__tracepoint_##name.key))		\
>  			__DO_TRACE(name,				\
>  				TP_ARGS(args),				\
> -				TP_CONDITION(cond), 0);			\
> +				TP_CONDITION(cond), 0, 1);		\
>  		if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) {		\
>  			WARN_ONCE(!rcu_is_watching(),			\
>  				  "RCU not watching for tracepoint");	\
> diff --git a/init/Kconfig b/init/Kconfig
> index fbd0cb06a50a..eedd0064fb36 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1984,6 +1984,7 @@ config BINDGEN_VERSION_TEXT
>  #
>  config TRACEPOINTS
>  	bool
> +	select TASKS_TRACE_RCU
>  
>  source "kernel/Kconfig.kexec"
>