Re: [patch V4 part 3 11/29] rcu: Provide rcu_irq_exit_preempt()

[Joel Fernandes]

Hi Thomas,

On Tue, May 05, 2020 at 03:44:05PM +0200, Thomas Gleixner wrote:

Thank you for CC'ing me.

> Interrupts and exceptions invoke rcu_irq_enter() on entry and need to
> invoke rcu_irq_exit() before they either return to the interrupted code or
> invoke the scheduler due to preemption.
> 
> The general assumption is that RCU idle code has to have preemption
> disabled so that a return from interrupt cannot schedule. So the return
> from interrupt code invokes rcu_irq_exit() and preempt_schedule_irq().
> 
> If there is any imbalance in the rcu_irq/nmi* invocations or RCU idle code
> had preemption enabled then this goes unnoticed until the CPU goes idle or
> some other RCU check is executed.
> 
> Provide rcu_irq_exit_preempt() which can be invoked from the
> interrupt/exception return code in case that preemption is enabled. It
> invokes rcu_irq_exit() and contains a few sanity checks in case that
> CONFIG_PROVE_RCU is enabled to catch such issues directly.

Could you let me know which patch or part in the multi-part series is using it?

> 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> ---
>  include/linux/rcutiny.h |    1 +
>  include/linux/rcutree.h |    1 +
>  kernel/rcu/tree.c       |   21 +++++++++++++++++++++
>  3 files changed, 23 insertions(+)
> 
> --- a/include/linux/rcutiny.h
> +++ b/include/linux/rcutiny.h
> @@ -71,6 +71,7 @@ static inline void rcu_irq_enter(void) {
>  static inline void rcu_irq_exit_irqson(void) { }
>  static inline void rcu_irq_enter_irqson(void) { }
>  static inline void rcu_irq_exit(void) { }
> +static inline void rcu_irq_exit_preempt(void) { }
>  static inline void exit_rcu(void) { }
>  static inline bool rcu_preempt_need_deferred_qs(struct task_struct *t)
>  {
> --- a/include/linux/rcutree.h
> +++ b/include/linux/rcutree.h
> @@ -46,6 +46,7 @@ void rcu_idle_enter(void);
>  void rcu_idle_exit(void);
>  void rcu_irq_enter(void);
>  void rcu_irq_exit(void);
> +void rcu_irq_exit_preempt(void);
>  void rcu_irq_enter_irqson(void);
>  void rcu_irq_exit_irqson(void);
>  
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -706,6 +706,27 @@ void noinstr rcu_irq_exit(void)
>  	rcu_nmi_exit();
>  }
>  
> +/**
> + * rcu_irq_exit_preempt - Inform RCU that current CPU is exiting irq
> + *			  towards in kernel preemption
> + *
> + * Same as rcu_irq_exit() but has a sanity check that scheduling is safe
> + * from RCU point of view. Invoked from return from interrupt before kernel
> + * preemption.
> + */
> +void rcu_irq_exit_preempt(void)
> +{
> +	lockdep_assert_irqs_disabled();
> +	rcu_nmi_exit();
> +
> +	RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nesting) <= 0,
> +			 "RCU dynticks_nesting counter underflow/zero!");

Makes sense.

> +	RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 0,
> +			 "RCU dynticks_nmi_nesting counter underflow/zero!");

This new function will be called only from the outer-most IRQ that
interrupted kernel mode (process context). Right? If so, a better (more
specific) check for the second RCU_LOCKDEP_WARN above is:

RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) != DYNTICK_IRQ_NONIDLE,
			 "Bad RCU dynticks_nmi_nesting counter\n");

That will make sure, it is only called from outer-most rcu_irq_exit() and
interrupting kernel mode.

Or, if [1] is merged, then we could just combine the checks into one check.
	RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nesting) != 1,
			 "Bad RCU dynticks_nmi_nesting counter\n");

> +	RCU_LOCKDEP_WARN(rcu_dynticks_curr_cpu_in_eqs(),
> +			 "RCU in extended quiescent state!");

Makes sense.

BTW, I wonder if a better place to do this "don't enter scheduler while RCU
is not watching" is rcu_note_context_switch()...

thanks,

 - Joel
[1] https://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git/commit/?h=rcu-dynticks-may4-rebased&id=b48863c234295d8ec956b50f6cf5ae0a0269f48d

> +}
> +
>  /*
>   * Wrapper for rcu_irq_exit() where interrupts are enabled.
>   *
>