Re: [PATCH tip/core/rcu 04/15] rcu: Permit RCU_NONIDLE() to be usedfrom interrupt context

[Josh Triplett]

On Thu, Aug 30, 2012 at 11:56:17AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@xxxxxxxxxx>
> 
> There is a need to use RCU from interrupt context, but either before
> rcu_irq_enter() is called or after rcu_irq_exit() is called.  If the
> interrupt occurs from idle, then lockdep-RCU will complain about such
> uses, as they appear to be illegal uses of RCU from the idle loop.
> In other environments, RCU_NONIDLE() could be used to properly protect
> the use of RCU, but RCU_NONIDLE() currently cannot be invoked except
> from process context.
> 
> This commit therefore modifies RCU_NONIDLE() to permit its use more
> globally.
> 
> Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

Something seems wrong about this.  The addition of EXPORT_SYMBOL_GPL
suggests that such interrupt handlers might live in modules.  In what
situation might a module interrupt handler get called from the idle
loop, before rcu_irq_enter or after rcu_irq_exit, and need to know that
when using RCU?

- Josh Triplett

> ---
>  include/linux/rcupdate.h |    6 ++----
>  kernel/rcutiny.c         |    2 ++
>  kernel/rcutree.c         |    2 ++
>  3 files changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 115ead2..0fbbd52 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -210,14 +210,12 @@ extern void exit_rcu(void);
>   * to nest RCU_NONIDLE() wrappers, but the nesting level is currently
>   * quite limited.  If deeper nesting is required, it will be necessary
>   * to adjust DYNTICK_TASK_NESTING_VALUE accordingly.
> - *
> - * This macro may be used from process-level code only.
>   */
>  #define RCU_NONIDLE(a) \
>  	do { \
> -		rcu_idle_exit(); \
> +		rcu_irq_enter(); \
>  		do { a; } while (0); \
> -		rcu_idle_enter(); \
> +		rcu_irq_exit(); \
>  	} while (0)
>  
>  /*
> diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
> index e4163c5..2e073a2 100644
> --- a/kernel/rcutiny.c
> +++ b/kernel/rcutiny.c
> @@ -115,6 +115,7 @@ void rcu_irq_exit(void)
>  	rcu_idle_enter_common(newval);
>  	local_irq_restore(flags);
>  }
> +EXPORT_SYMBOL_GPL(rcu_irq_exit);
>  
>  /* Common code for rcu_idle_exit() and rcu_irq_enter(), see kernel/rcutree.c. */
>  static void rcu_idle_exit_common(long long oldval)
> @@ -172,6 +173,7 @@ void rcu_irq_enter(void)
>  	rcu_idle_exit_common(oldval);
>  	local_irq_restore(flags);
>  }
> +EXPORT_SYMBOL_GPL(rcu_irq_enter);
>  
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index f280e54..96b8aff 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -447,6 +447,7 @@ void rcu_irq_exit(void)
>  		rcu_idle_enter_common(rdtp, oldval);
>  	local_irq_restore(flags);
>  }
> +EXPORT_SYMBOL_GPL(rcu_irq_exit);
>  
>  /*
>   * rcu_idle_exit_common - inform RCU that current CPU is moving away from idle
> @@ -542,6 +543,7 @@ void rcu_irq_enter(void)
>  		rcu_idle_exit_common(rdtp, oldval);
>  	local_irq_restore(flags);
>  }
> +EXPORT_SYMBOL_GPL(rcu_irq_enter);
>  
>  /**
>   * rcu_nmi_enter - inform RCU of entry to NMI context
> -- 
> 1.7.8
> 
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