rcu: Suppress smp_processor_id() complaint in synchronize_rcu_expedited_wait()

The normal grace period's RCU CPU stall warnings are invoked from the
scheduling-clock interrupt handler, and can thus invoke smp_processor_id()
with impunity, which allows them to directly invoke dump_cpu_task().
In contrast, the expedited grace period's RCU CPU stall warnings are
invoked from process context, which causes the dump_cpu_task() function's
calls to smp_processor_id() to complain bitterly in debug kernels.

This commit therefore causes synchronize_rcu_expedited_wait() to disable
preemption around its call to dump_cpu_task().

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index ed6c3cc..927abaf 100644 (file)
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -667,7 +667,9 @@ static void synchronize_rcu_expedited_wait(void)
                                mask = leaf_node_cpu_bit(rnp, cpu);
                                if (!(READ_ONCE(rnp->expmask) & mask))
                                        continue;
+                               preempt_disable(); // For smp_processor_id() in dump_cpu_task().
                                dump_cpu_task(cpu);
+                               preempt_enable();
                        }
                }
                jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;