rcu: Complain if blocking in preemptible RCU read-side critical section

Although preemptible RCU allows its read-side critical sections to be
preempted, general blocking is forbidden.  The reason for this is that
excessive preemption times can be handled by CONFIG_RCU_BOOST=y, but a
voluntarily blocked task doesn't care how high you boost its priority.
Because preemptible RCU is a global mechanism, one ill-behaved reader
hurts everyone.  Hence the prohibition against general blocking in
RCU-preempt read-side critical sections.  Preemption yes, blocking no.

This commit enforces this prohibition.

There is a special exception for the -rt patchset (which they kindly
volunteered to implement):  It is OK to block (as opposed to merely being
preempted) within an RCU-preempt read-side critical section, but only if
the blocking is subject to priority inheritance.  This exception permits
CONFIG_RCU_BOOST=y to get -rt RCU readers out of trouble.

Why doesn't this exception also apply to mainline's rt_mutex?  Because
of the possibility that someone does general blocking while holding
an rt_mutex.  Yes, the priority boosting will affect the rt_mutex,
but it won't help with the task doing general blocking while holding
that rt_mutex.
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -478,7 +478,7 @@ void rcu_note_context_switch(bool preempt)
 	barrier(); /* Avoid RCU read-side critical sections leaking down. */
 	trace_rcu_utilization(TPS("Start context switch"));
 	rcu_sched_qs();
-	rcu_preempt_note_context_switch();
+	rcu_preempt_note_context_switch(preempt);
 	/* Load rcu_urgent_qs before other flags. */
 	if (!smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs)))
 		goto out;

kernel/rcu/tree.h

@@ -477,7 +477,7 @@ DECLARE_PER_CPU(char, rcu_cpu_has_work);
 
 /* Forward declarations for rcutree_plugin.h */
 static void rcu_bootup_announce(void);
-static void rcu_preempt_note_context_switch(void);
+static void rcu_preempt_note_context_switch(bool preempt);
 static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp);
 #ifdef CONFIG_HOTPLUG_CPU
 static bool rcu_preempt_has_tasks(struct rcu_node *rnp);

kernel/rcu/tree_plugin.h

@@ -286,12 +286,13 @@ static void rcu_preempt_qs(void)
  *
  * Caller must disable interrupts.
  */
-static void rcu_preempt_note_context_switch(void)
+static void rcu_preempt_note_context_switch(bool preempt)
 {
 	struct task_struct *t = current;
 	struct rcu_data *rdp;
 	struct rcu_node *rnp;
 
+	WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0);
 	if (t->rcu_read_lock_nesting > 0 &&
 	    !t->rcu_read_unlock_special.b.blocked) {
 
@@ -738,7 +739,7 @@ static void __init rcu_bootup_announce(void)
  * Because preemptible RCU does not exist, we never have to check for
  * CPUs being in quiescent states.
  */
-static void rcu_preempt_note_context_switch(void)
+static void rcu_preempt_note_context_switch(bool preempt)
 {
 }