rcu: Disallow callback registry on offline CPUs (0d8ee37e) · Commits · 戴 / test

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> Reviewed-by:

Josh Triplett <josh@joshtriplett.org>

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);

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