[PATCH 2/3] rcutorture: Synchronously wait for all rcu_torture_irq() callbacks to complete

["Paul E. McKenney" <paulmck@kernel.org>]

From: Zqiang <qiang.zhang@linux.dev>

The rcu_torture_reader() drives RCU readers from interrupt context via
smp_call_function_single(cpu, rcu_torture_irq, NULL, 0) with wait=0, to
runs rcu_torture_irq() on a remote CPU. this is async, nothing waits for
the remote handler to run.

On shutdown, torture_stop_kthread() only waits for each reader kthread to
return, and the reader's timer_delete_sync() only drains its timer. Neither
waits for a rcu_torture_irq() which still pending or executing on a remote
CPU, so it can run after all readers have exited and rcu_torture_cleanup()
has already advanced.

1. rcu_torture_irq() may issue cur_ops->call(rhp, rcu_torture_timer_cb)
   after cur_ops->cb_barrier() has been waiting for all outstanding
   callbacks complete. once the module is unloaded, fires into freed
   module text, a use-after-free happen.

2. rcu_torture_irq() may still be inside rcu_torture_one_read(), holding
   a read-side critical section, when cur_ops->cleanup() tears the flavor
   down (e.g. cleanup_srcu_struct()), triggering an active-reader warning
   or use-after-free of the torn-down structure.

This commit therefore issue a kick_all_cpus_sync() after all readers
kthread have returned and before cur_ops->cb_barrier(), synchronous IPI
round trip to every CPU guarantees that every rcu_torture_irq() which
previously issued by any reader has completed.

Signed-off-by: Zqiang <qiang.zhang@linux.dev>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/rcutorture.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 79807475b672..51ed35f5aab1 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -4491,6 +4491,8 @@ rcu_torture_cleanup(void)
 		for (i = 0; i < nrealreaders; i++)
 			torture_stop_kthread(rcu_torture_reader,
 					     reader_tasks[i]);
+		if (irqreader && cur_ops->irq_capable)
+			kick_all_cpus_sync();
 		kfree(reader_tasks);
 		reader_tasks = NULL;
 	}
-- 
2.40.1