rcu-tasks: Track blocked RCU Tasks Trace readers

This commit places any task that has ever blocked within its current
RCU Tasks Trace read-side critical 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.  The purpose of this
commit is to provide the information needed to eliminate the current
scan of the full task list.

This commit offsets the INT_MIN value for ->trc_reader_nesting with the
new nesting level in order to avoid queueing tasks that are exiting
their read-side critical sections.

[ paulmck: Apply kernel test robot feedback. ]
[ paulmck: Apply feedback from syzbot+9bb26e7c5e8e4fa7e641@syzkaller.appspotmail.com ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: syzbot <syzbot+9bb26e7c5e8e4fa7e641@syzkaller.appspotmail.com>
Tested-by: "Zhang, Qiang1" <qiang1.zhang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.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

@@ -174,12 +174,19 @@ void synchronize_rcu_tasks(void);
 #define TRC_NEED_QS_CHECKED	0x2  // Task has been checked for needing quiescent state.
 
 u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new);
+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_special.b.need_qs)) &&	\
-		    likely(!READ_ONCE((t)->trc_reader_nesting)))		\
+		    likely(!___rttq_nesting)) {					\
 			rcu_trc_cmpxchg_need_qs((t), 0,	TRC_NEED_QS_CHECKED);	\
+		} else if (___rttq_nesting && ___rttq_nesting != INT_MIN &&	\
+			   !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)
@@ -188,7 +195,7 @@ u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new);
 #define rcu_tasks_qs(t, preempt)					\
 do {									\
 	rcu_tasks_classic_qs((t), (preempt));				\
-	rcu_tasks_trace_qs((t));					\
+	rcu_tasks_trace_qs(t);						\
 } while (0)
 
 # ifdef CONFIG_TASKS_RUDE_RCU

include/linux/rcupdate_trace.h

@@ -75,7 +75,7 @@ static inline void rcu_read_unlock_trace(void)
 	nesting = READ_ONCE(t->trc_reader_nesting) - 1;
 	barrier(); // Critical section before disabling.
 	// Disable IPI-based setting of .need_qs.
-	WRITE_ONCE(t->trc_reader_nesting, INT_MIN);
+	WRITE_ONCE(t->trc_reader_nesting, INT_MIN + nesting);
 	if (likely(!READ_ONCE(t->trc_reader_special.s)) || nesting) {
 		WRITE_ONCE(t->trc_reader_nesting, nesting);
 		return;  // We assume shallow reader nesting.

kernel/rcu/tasks.h

@@ -1261,6 +1261,24 @@ 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);
+}
+EXPORT_SYMBOL_GPL(rcu_tasks_trace_qs_blkd);
+
 /* 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)
 {
@@ -1586,9 +1604,11 @@ 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);
+
 	rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
 	WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
-	if (WARN_ON_ONCE(rcu_ld_need_qs(t) & TRC_NEED_QS))
+	if (WARN_ON_ONCE(rcu_ld_need_qs(t) & TRC_NEED_QS) || trs.b.blocked)
 		rcu_read_unlock_trace_special(t);
 	else
 		WRITE_ONCE(t->trc_reader_nesting, 0);