Re: [PATCH v4 2/5] x86, traps: Track entry into and exit from IST context

[Lai Jiangshan <laijs@cn.fujitsu.com>]

> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 



Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>


> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8749f43f3f05..fc0236992655 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -759,39 +759,71 @@ void rcu_irq_enter(void)
>  /**
>   * rcu_nmi_enter - inform RCU of entry to NMI context
>   *
> - * If the CPU was idle with dynamic ticks active, and there is no
> - * irq handler running, this updates rdtp->dynticks_nmi to let the
> - * RCU grace-period handling know that the CPU is active.
> + * If the CPU was idle from RCU's viewpoint, update rdtp->dynticks and
> + * rdtp->dynticks_nmi_nesting to let the RCU grace-period handling know
> + * that the CPU is active.  This implementation permits nested NMIs, as
> + * long as the nesting level does not overflow an int.  (You will probably
> + * run out of stack space first.)
>   */
>  void rcu_nmi_enter(void)
>  {
>  	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> +	int incby = 2;
>  
> -	if (rdtp->dynticks_nmi_nesting == 0 &&
> -	    (atomic_read(&rdtp->dynticks) & 0x1))
> -		return;
> -	rdtp->dynticks_nmi_nesting++;
> -	smp_mb__before_atomic();  /* Force delay from prior write. */
> -	atomic_inc(&rdtp->dynticks);
> -	/* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
> -	smp_mb__after_atomic();  /* See above. */
> -	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
> +	/* Complain about underflow. */
> +	WARN_ON_ONCE(rdtp->dynticks_nmi_nesting < 0);
> +
> +	/*
> +	 * If idle from RCU viewpoint, atomically increment ->dynticks
> +	 * to mark non-idle and increment ->dynticks_nmi_nesting by one.
> +	 * Otherwise, increment ->dynticks_nmi_nesting by two.  This means
> +	 * if ->dynticks_nmi_nesting is equal to one, we are guaranteed
> +	 * to be in the outermost NMI handler that interrupted an RCU-idle
> +	 * period (observation due to Andy Lutomirski).
> +	 */
> +	if (!(atomic_read(&rdtp->dynticks) & 0x1)) {
> +		smp_mb__before_atomic();  /* Force delay from prior write. */
> +		atomic_inc(&rdtp->dynticks);
> +		/* atomic_inc() before later RCU read-side crit sects */
> +		smp_mb__after_atomic();  /* See above. */
> +		WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
> +		incby = 1;
> +	}
> +	rdtp->dynticks_nmi_nesting += incby;

I prefer a "else" branch here.

	if (!(atomic_read(&rdtp->dynticks) & 0x1)) {
		...
		WARN_ON_ONCE(rdtp->dynticks_nmi_nesting); /* paired with "rdtp->dynticks_nmi_nesting = 0" */
		rdtp->dynticks_nmi_nesting = 1;  /* paired with "if (rdtp->dynticks_nmi_nesting != 1) {" */
	} else {
		rdtp->dynticks_nmi_nesting += 2;
	}



> +	barrier();
>  }
>  
>  /**
>   * rcu_nmi_exit - inform RCU of exit from NMI context
>   *
> - * If the CPU was idle with dynamic ticks active, and there is no
> - * irq handler running, this updates rdtp->dynticks_nmi to let the
> - * RCU grace-period handling know that the CPU is no longer active.
> + * If we are returning from the outermost NMI handler that interrupted an
> + * RCU-idle period, update rdtp->dynticks and rdtp->dynticks_nmi_nesting
> + * to let the RCU grace-period handling know that the CPU is back to
> + * being RCU-idle.
>   */
>  void rcu_nmi_exit(void)
>  {
>  	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
>  
> -	if (rdtp->dynticks_nmi_nesting == 0 ||
> -	    --rdtp->dynticks_nmi_nesting != 0)
> +	/*
> +	 * Check for ->dynticks_nmi_nesting underflow and bad ->dynticks.
> +	 * (We are exiting an NMI handler, so RCU better be paying attention
> +	 * to us!)
> +	 */
> +	WARN_ON_ONCE(rdtp->dynticks_nmi_nesting <= 0);
> +	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
> +
> +	/*
> +	 * If the nesting level is not 1, the CPU wasn't RCU-idle, so
> +	 * leave it in non-RCU-idle state.
> +	 */
> +	if (rdtp->dynticks_nmi_nesting != 1) {
> +		rdtp->dynticks_nmi_nesting -= 2;
>  		return;
> +	}
> +
> +	/* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
> +	rdtp->dynticks_nmi_nesting = 0;
>  	/* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
>  	smp_mb__before_atomic();  /* See above. */
>  	atomic_inc(&rdtp->dynticks);
> 
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