Re: [RFC PATCH tip/master] kprobes: Use synchronize_rcu_tasks() for optprobe wit CONFIG_PREEMPT

[Masami Hiramatsu]

On Thu, 25 May 2017 08:14:01 -0700
"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

> On Thu, May 25, 2017 at 08:15:55AM +0200, Ingo Molnar wrote:
> > 
> > * Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> > 
> > > --- a/kernel/kprobes.c
> > > +++ b/kernel/kprobes.c
> > > @@ -377,6 +377,23 @@ static inline void copy_kprobe(struct kprobe *ap, struct kprobe *p)
> > >  static bool kprobes_allow_optimization;
> > >  
> > >  /*
> > > + * Synchronizing wait on trampline code for interrupted tasks/threads.
> > > + * Since the threads running on dynamically allocated trampline code
> > > + * can be interrupted, kprobes has to wait for those tasks back on
> > > + * track and scheduled. If the kernel is preemptive, the thread can be
> > > + * preempted by other tasks on the trampoline too. For such case, this
> > > + * calls synchronize_rcu_tasks() to wait for those tasks back on track.
> > > + */
> > > +static void synchronize_on_trampoline(void)
> > > +{
> > > +#ifdef CONFIG_PREEMPT
> > > +	synchronize_rcu_tasks();
> > > +#else
> > > +	synchronize_sched();
> > > +#endif
> > > +}
> > 
> > So that's really unacceptably ugly.
> > 
> > Paul, I still question the need to have tasks-RCU as a Kconfig distinction, 
> > _especially_ if its API usage results in such ugly secondary #ifdefs...
> > 
> > Why isn't there a single synchronize_rcu_tasks() API function, which does what is 
> > expected, where the _RCU_ code figures out how to implement it?
> > 
> > I.e.:
> > 
> >  - There should be no user configurable TASKS_RCU Kconfig setting - at most a
> >    helper Kconfig that is automatically selected by the RCU code itself.
> > 
> >  - Both ftrace andkprobes would use a simple synchronize_rcu_tasks() call.
> > 
> > Thanks,
> 
> How about the following (untested) patch?

This patch is good to me for kprobes at least.

Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>

Steve, how about ftrace usage?

Thank you,

> 
> This is against -rcu's rcu/dev branch, FWIW.
> 
> And I am also queuing patches with other cleanups, including do_exit(),
> enabled by this approach.
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 9ab47fba45cea06e223e524d392621b64c174720
> Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Date:   Thu May 25 08:05:00 2017 -0700
> 
>     rcu: Drive TASKS_RCU directly off of PREEMPT
>     
>     The actual use of TASKS_RCU is only when PREEMPT, otherwise RCU-sched
>     is used instead.  This commit therefore makes synchronize_rcu_tasks()
>     and call_rcu_tasks() available always, but mapped to synchronize_sched()
>     and call_rcu_sched(), respectively, when !PREEMPT.  This approach also
>     allows some #ifdefs to be removed from rcutorture.
>     
>     Reported-by: Ingo Molnar <mingo@xxxxxxxxxx>
>     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> ---
> 
>  include/linux/rcupdate.h |    6 ++++--
>  kernel/rcu/Kconfig       |    3 +--
>  kernel/rcu/rcutorture.c  |   17 +----------------
>  3 files changed, 6 insertions(+), 20 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index f816fc72b51e..c3f380befdd7 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -58,8 +58,6 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func);
>  void call_rcu_bh(struct rcu_head *head, rcu_callback_t func);
>  void call_rcu_sched(struct rcu_head *head, rcu_callback_t func);
>  void synchronize_sched(void);
> -void call_rcu_tasks(struct rcu_head *head, rcu_callback_t func);
> -void synchronize_rcu_tasks(void);
>  void rcu_barrier_tasks(void);
>  
>  #ifdef CONFIG_PREEMPT_RCU
> @@ -176,10 +174,14 @@ extern struct srcu_struct tasks_rcu_exit_srcu;
>  		rcu_all_qs(); \
>  		rcu_note_voluntary_context_switch_lite(t); \
>  	} while (0)
> +void call_rcu_tasks(struct rcu_head *head, rcu_callback_t func);
> +void synchronize_rcu_tasks(void);
>  #else /* #ifdef CONFIG_TASKS_RCU */
>  #define TASKS_RCU(x) do { } while (0)
>  #define rcu_note_voluntary_context_switch_lite(t)	do { } while (0)
>  #define rcu_note_voluntary_context_switch(t)		rcu_all_qs()
> +#define call_rcu_tasks call_rcu_sched
> +#define synchronize_rcu_tasks synchronize_sched
>  #endif /* #else #ifdef CONFIG_TASKS_RCU */
>  
>  /**
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index be90c945063f..9210379c0353 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -69,8 +69,7 @@ config TREE_SRCU
>  	  This option selects the full-fledged version of SRCU.
>  
>  config TASKS_RCU
> -	bool
> -	default n
> +	def_bool PREEMPT
>  	select SRCU
>  	help
>  	  This option enables a task-based RCU implementation that uses
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index aedc8f2ad955..273032dc8f2d 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -659,8 +659,6 @@ static struct rcu_torture_ops sched_ops = {
>  	.name		= "sched"
>  };
>  
> -#ifdef CONFIG_TASKS_RCU
> -
>  /*
>   * Definitions for RCU-tasks torture testing.
>   */
> @@ -698,24 +696,11 @@ static struct rcu_torture_ops tasks_ops = {
>  	.name		= "tasks"
>  };
>  
> -#define RCUTORTURE_TASKS_OPS &tasks_ops,
> -
>  static bool __maybe_unused torturing_tasks(void)
>  {
>  	return cur_ops == &tasks_ops;
>  }
>  
> -#else /* #ifdef CONFIG_TASKS_RCU */
> -
> -#define RCUTORTURE_TASKS_OPS
> -
> -static bool __maybe_unused torturing_tasks(void)
> -{
> -	return false;
> -}
> -
> -#endif /* #else #ifdef CONFIG_TASKS_RCU */
> -
>  /*
>   * RCU torture priority-boost testing.  Runs one real-time thread per
>   * CPU for moderate bursts, repeatedly registering RCU callbacks and
> @@ -1712,7 +1697,7 @@ rcu_torture_init(void)
>  	int firsterr = 0;
>  	static struct rcu_torture_ops *torture_ops[] = {
>  		&rcu_ops, &rcu_bh_ops, &rcu_busted_ops, &srcu_ops, &srcud_ops,
> -		&sched_ops, RCUTORTURE_TASKS_OPS
> +		&sched_ops, &tasks_ops,
>  	};
>  
>  	if (!torture_init_begin(torture_type, verbose, &torture_runnable))
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>