tasks-rcu: Track blocked RCU Tasks Trace readers

This commit places any task that has ever blocked within an RCU
Tasks Trace read-side critcial section on a per-CPU list within the
rcu_tasks_percpu structure.  Tasks are removed from this list when they
exit by the exit_tasks_rcu_finish_trace() function.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: KP Singh <kpsingh@kernel.org>

include/linux/rcupdate.h

@@ -170,13 +170,19 @@ void synchronize_rcu_tasks(void);
 # endif
 
 # ifdef CONFIG_TASKS_TRACE_RCU
-# define rcu_tasks_trace_qs(t)						\
-	do {								\
-		if (!likely(READ_ONCE((t)->trc_reader_checked)) &&	\
-		    !unlikely(READ_ONCE((t)->trc_reader_nesting))) {	\
-			smp_store_release(&(t)->trc_reader_checked, true); \
-			smp_mb(); /* Readers partitioned by store. */	\
-		}							\
+void rcu_tasks_trace_qs_blkd(struct task_struct *t);
+# define rcu_tasks_trace_qs(t)							\
+	do {									\
+		int ___rttq_nesting = READ_ONCE((t)->trc_reader_nesting);	\
+										\
+		if (!likely(READ_ONCE((t)->trc_reader_checked)) &&		\
+		    !unlikely(___rttq_nesting)) {				\
+			smp_store_release(&(t)->trc_reader_checked, true);	\
+			smp_mb(); /* Readers partitioned by store. */		\
+		} else if (___rttq_nesting &&					\
+			   !READ_ONCE((t)->trc_reader_special.b.blocked)) {	\
+			rcu_tasks_trace_qs_blkd(t);				\
+		}								\
 	} while (0)
 # else
 # define rcu_tasks_trace_qs(t) do { } while (0)

kernel/rcu/tasks.h

@@ -1239,6 +1239,23 @@ void rcu_read_unlock_trace_special(struct task_struct *t)
 }
 EXPORT_SYMBOL_GPL(rcu_read_unlock_trace_special);
 
+/* Add a newly blocked reader task to its CPU's list. */
+void rcu_tasks_trace_qs_blkd(struct task_struct *t)
+{
+	unsigned long flags;
+	struct rcu_tasks_percpu *rtpcp;
+
+	local_irq_save(flags);
+	rtpcp = this_cpu_ptr(rcu_tasks_trace.rtpcpu);
+	raw_spin_lock_rcu_node(rtpcp); // irqs already disabled
+	t->trc_blkd_cpu = smp_processor_id();
+	if (!rtpcp->rtp_blkd_tasks.next)
+		INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
+	list_add(&t->trc_blkd_node, &rtpcp->rtp_blkd_tasks);
+	t->trc_reader_special.b.blocked = true;
+	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+}
+
 /* Add a task to the holdout list, if it is not already on the list. */
 static void trc_add_holdout(struct task_struct *t, struct list_head *bhp)
 {
@@ -1599,10 +1616,12 @@ static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
 /* Report any needed quiescent state for this exiting task. */
 static void exit_tasks_rcu_finish_trace(struct task_struct *t)
 {
+	union rcu_special trs = READ_ONCE(t->trc_reader_special);
+
 	WRITE_ONCE(t->trc_reader_checked, true);
 	WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
 	WRITE_ONCE(t->trc_reader_nesting, 0);
-	if (WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs)))
+	if (WARN_ON_ONCE(trs.b.need_qs) || trs.b.blocked)
 		rcu_read_unlock_trace_special(t);
 }
 

kernel/rcu/tree_plugin.h

@@ -899,8 +899,8 @@ void rcu_note_context_switch(bool preempt)
 	this_cpu_write(rcu_data.rcu_urgent_qs, false);
 	if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs)))
 		rcu_momentary_dyntick_idle();
-	rcu_tasks_qs(current, preempt);
 out:
+	rcu_tasks_qs(current, preempt);
 	trace_rcu_utilization(TPS("End context switch"));
 }
 EXPORT_SYMBOL_GPL(rcu_note_context_switch);