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Commit 0d8ee37e authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Paul E. McKenney
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rcu: Disallow callback registry on offline CPUs 



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: default avatarPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: default avatarJosh Triplett <josh@joshtriplett.org>
parent 1331e7a1

kernel/rcutree.c

+10 −0
Original line number Diff line number Diff line
@@ -1505,6 +1505,9 @@ 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 */
@@ -1927,6 +1930,12 @@ __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);

		local_irq_restore(flags);

		return;

	}

	ACCESS_ONCE(rdp->qlen)++;

	if (lazy)

		rdp->qlen_lazy++;
@@ -2464,6 +2473,7 @@ 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);