[PATCH tip/core/rcu 30/30] rcu: Make rcu_barrier() account for offline no-CBs CPUs

[paulmck]

From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>

Currently, rcu_barrier() ignores offline CPUs,  However, it is possible
for an offline no-CBs CPU to have callbacks queued, and rcu_barrier()
must wait for those callbacks.  This commit therefore makes rcu_barrier()
directly invoke the rcu_barrier_func() with interrupts disabled for such
CPUs.  This requires passing the CPU number into this function so that
it can entrain the rcu_barrier() callback onto the correct CPU's callback
list, given that the code must instead execute on the current CPU.

While in the area, this commit fixes a bug where the first CPU's callback
might have been invoked before rcu_segcblist_entrain() returned, which
would also result in an early wakeup.

Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs")
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 5.5.x
---
 include/trace/events/rcu.h |  1 +
 kernel/rcu/tree.c          | 32 ++++++++++++++++++++------------
 2 files changed, 21 insertions(+), 12 deletions(-)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 5e49b06..d56d54c 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -712,6 +712,7 @@ TRACE_EVENT_RCU(rcu_torture_read,
  *	"Begin": rcu_barrier() started.
  *	"EarlyExit": rcu_barrier() piggybacked, thus early exit.
  *	"Inc1": rcu_barrier() piggyback check counter incremented.
+ *	"OfflineNoCBQ": rcu_barrier() found offline no-CBs CPU with callbacks.
  *	"OnlineQ": rcu_barrier() found online CPU with callbacks.
  *	"OnlineNQ": rcu_barrier() found online CPU, no callbacks.
  *	"IRQ": An rcu_barrier_callback() callback posted on remote CPU.
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d15041f..160643e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3098,9 +3098,10 @@ static void rcu_barrier_callback(struct rcu_head *rhp)
 /*
  * Called with preemption disabled, and from cross-cpu IRQ context.
  */
-static void rcu_barrier_func(void *unused)
+static void rcu_barrier_func(void *cpu_in)
 {
-	struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
+	uintptr_t cpu = (uintptr_t)cpu_in;
+	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 
 	rcu_barrier_trace(TPS("IRQ"), -1, rcu_state.barrier_sequence);
 	rdp->barrier_head.func = rcu_barrier_callback;
@@ -3127,7 +3128,7 @@ static void rcu_barrier_func(void *unused)
  */
 void rcu_barrier(void)
 {
-	int cpu;
+	uintptr_t cpu;
 	struct rcu_data *rdp;
 	unsigned long s = rcu_seq_snap(&rcu_state.barrier_sequence);
 
@@ -3150,13 +3151,14 @@ void rcu_barrier(void)
 	rcu_barrier_trace(TPS("Inc1"), -1, rcu_state.barrier_sequence);
 
 	/*
-	 * Initialize the count to one rather than to zero in order to
-	 * avoid a too-soon return to zero in case of a short grace period
-	 * (or preemption of this task).  Exclude CPU-hotplug operations
-	 * to ensure that no offline CPU has callbacks queued.
+	 * Initialize the count to two rather than to zero in order
+	 * to avoid a too-soon return to zero in case of an immediate
+	 * invocation of the just-enqueued callback (or preemption of
+	 * this task).  Exclude CPU-hotplug operations to ensure that no
+	 * offline non-offloaded CPU has callbacks queued.
 	 */
 	init_completion(&rcu_state.barrier_completion);
-	atomic_set(&rcu_state.barrier_cpu_count, 1);
+	atomic_set(&rcu_state.barrier_cpu_count, 2);
 	get_online_cpus();
 
 	/*
@@ -3166,13 +3168,19 @@ void rcu_barrier(void)
 	 */
 	for_each_possible_cpu(cpu) {
 		rdp = per_cpu_ptr(&rcu_data, cpu);
-		if (!cpu_online(cpu) &&
+		if (cpu_is_offline(cpu) &&
 		    !rcu_segcblist_is_offloaded(&rdp->cblist))
 			continue;
-		if (rcu_segcblist_n_cbs(&rdp->cblist)) {
+		if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_online(cpu)) {
 			rcu_barrier_trace(TPS("OnlineQ"), cpu,
 					  rcu_state.barrier_sequence);
-			smp_call_function_single(cpu, rcu_barrier_func, NULL, 1);
+			smp_call_function_single(cpu, rcu_barrier_func, (void *)cpu, 1);
+		} else if (cpu_is_offline(cpu)) {
+			rcu_barrier_trace(TPS("OfflineNoCBQ"), cpu,
+					  rcu_state.barrier_sequence);
+			local_irq_disable();
+			rcu_barrier_func((void *)cpu);
+			local_irq_enable();
 		} else {
 			rcu_barrier_trace(TPS("OnlineNQ"), cpu,
 					  rcu_state.barrier_sequence);
@@ -3184,7 +3192,7 @@ void rcu_barrier(void)
 	 * Now that we have an rcu_barrier_callback() callback on each
 	 * CPU, and thus each counted, remove the initial count.
 	 */
-	if (atomic_dec_and_test(&rcu_state.barrier_cpu_count))
+	if (atomic_sub_and_test(2, &rcu_state.barrier_cpu_count))
 		complete(&rcu_state.barrier_completion);
 
 	/* Wait for all rcu_barrier_callback() callbacks to be invoked. */
-- 
2.9.5