rcu: Add might_sleep() check to synchronize_rcu()

[Thomas Gleixner]

Subject: rcu: Add might_sleep() check to synchronize_rcu()
From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Date: Fri, 23 Mar 2018 22:02:18 +0100

Joel reported a debugobjects warning which is triggered by a RCU callback
invoking synchronize_rcu(). RCU callbacks run in softirq context, so
calling synchronize_rcu() is a bad idea as it might sleep.

debugobjects triggers because __wait_rcu_gp() uses on stack objects and
invokes debug_object_init_on_stack(). That function checks the object
address against current's task stack, which fails because the code runs on
the softirq stack.

synchronize_rcu() lacks a might_sleep() check which would have caught that
issue way earlier because it would trigger with the minimal debug options
enabled.

Add a might_sleep() check to catch such cases.

Reported-by: Joel Fernandes <joelaf@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
---
 kernel/rcu/tree_plugin.h |    1 +
 1 file changed, 1 insertion(+)

--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -753,6 +753,7 @@ void synchronize_rcu(void)
 			 "Illegal synchronize_rcu() in RCU read-side critical section");
 	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
 		return;
+	might_sleep();
 	if (rcu_gp_is_expedited())
 		synchronize_rcu_expedited();
 	else