Re: [PATCH tip/core/rcu 2/3] rcu: Disallow callback registry on offline CPUs

[Josh Triplett <josh@joshtriplett.org>]

On Thu, Aug 30, 2012 at 12:03:02PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@linaro.org>
> 
> Posting a callback after the CPU_DEAD notifier effectively leaks
> that callback unless/until that CPU comes back online.  Silence is
> unhelpful when attempting to track down such leaks, so this commit emits
> a WARN_ON_ONCE() and unconditionally leaks the callback when an offline
> CPU attempts to register a callback.  The rdp->nxttail[RCU_NEXT_TAIL] is
> set to NULL in the CPU_DEAD notifier and restored in the CPU_UP_PREPARE
> notifier, allowing _call_rcu() to determine exactly when posting callbacks
> is illegal.
> 
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

One suggestion below; with or without that change:

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

> ---
>  kernel/rcutree.c |   10 ++++++++++
>  1 files changed, 10 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 9854a00..5f8c4dd 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1503,6 +1503,9 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
>  	WARN_ONCE(rdp->qlen != 0 || rdp->nxtlist != NULL,
>  		  "rcu_cleanup_dead_cpu: Callbacks on offline CPU %d: qlen=%lu, nxtlist=%p\n",
>  		  cpu, rdp->qlen, rdp->nxtlist);
> +	init_callback_list(rdp);
> +	/* Disallow further callbacks on this CPU. */
> +	rdp->nxttail[RCU_NEXT_TAIL] = NULL;
>  }
>  
>  #else /* #ifdef CONFIG_HOTPLUG_CPU */
> @@ -1925,6 +1928,12 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
>  	rdp = this_cpu_ptr(rsp->rda);
>  
>  	/* Add the callback to our list. */
> +	if (unlikely(rdp->nxttail[RCU_NEXT_TAIL] == NULL)) {
> +		/* _call_rcu() is illegal on offline CPU; leak the callback. */
> +		WARN_ON_ONCE(1);

You can write this as:

if (WARN_ON_ONCE(rdp->nxttail[RCU_NEXT_TAIL] == NULL))

WARN_ON_ONCE also has a built-in unlikely() already.

> +		local_irq_restore(flags);
> +		return;
> +	}
>  	ACCESS_ONCE(rdp->qlen)++;
>  	if (lazy)
>  		rdp->qlen_lazy++;
> @@ -2462,6 +2471,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
>  	rdp->qlen_last_fqs_check = 0;
>  	rdp->n_force_qs_snap = rsp->n_force_qs;
>  	rdp->blimit = blimit;
> +	init_callback_list(rdp);  /* Re-enable callbacks on this CPU. */
>  	rdp->dynticks->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
>  	atomic_set(&rdp->dynticks->dynticks,
>  		   (atomic_read(&rdp->dynticks->dynticks) & ~0x1) + 1);
> -- 
> 1.7.8
>