[PATCH] rcu/nocb: Fix the WARN_ON_ONCE() in rcu_nocb_rdp_deoffload()

[Zqiang <qiang.zhang1211@gmail.com>]

Currently, running rcutorture test with torture_type=rcu fwd_progress=8
n_barrier_cbs=8 nocbs_nthreads=8 nocbs_toggle=100 onoff_interval=60
test_boost=2, will trigger the following warning:

WARNING: CPU: 19 PID: 100 at kernel/rcu/tree_nocb.h:1061 rcu_nocb_rdp_deoffload+0x292/0x2a0
RIP: 0010:rcu_nocb_rdp_deoffload+0x292/0x2a0
[18839.537322] Call Trace:
[18839.538006]  <TASK>
[18839.538596]  ? __warn+0x7e/0x120
[18839.539491]  ? rcu_nocb_rdp_deoffload+0x292/0x2a0
[18839.540757]  ? report_bug+0x18e/0x1a0
[18839.541805]  ? handle_bug+0x3d/0x70
[18839.542837]  ? exc_invalid_op+0x18/0x70
[18839.543959]  ? asm_exc_invalid_op+0x1a/0x20
[18839.545165]  ? rcu_nocb_rdp_deoffload+0x292/0x2a0
[18839.546547]  rcu_nocb_cpu_deoffload+0x70/0xa0
[18839.547814]  rcu_nocb_toggle+0x136/0x1c0
[18839.548960]  ? __pfx_rcu_nocb_toggle+0x10/0x10
[18839.550073]  kthread+0xd1/0x100
[18839.550958]  ? __pfx_kthread+0x10/0x10
[18839.552008]  ret_from_fork+0x2f/0x50
[18839.553002]  ? __pfx_kthread+0x10/0x10
[18839.553968]  ret_from_fork_asm+0x1a/0x30
[18839.555038]  </TASK>

CPU0                               CPU2                          CPU3
//rcu_nocb_toggle             //nocb_cb_wait                   //rcutorture

// deoffload CPU1             // process CPU1's rdp
rcu_barrier()
    rcu_segcblist_entrain()
        rcu_segcblist_add_len(1);
        // len == 2
        // enqueue barrier
        // callback to CPU1's
        // rdp->cblist
                             rcu_do_batch()
                                 // invoke CPU1's rdp->cblist
                                 // callback
                                 rcu_barrier_callback()
                                                             rcu_barrier()
                                                               mutex_lock(&rcu_state.barrier_mutex);
                                                               // still see len == 2
                                                               // enqueue barrier callback
                                                               // to CPU1's rdp->cblist
                                                               rcu_segcblist_entrain()
                                                                   rcu_segcblist_add_len(1);
                                                                   // len == 3
                                 // decrement len
                                 rcu_segcblist_add_len(-2);
                             kthread_parkme()

// CPU1's rdp->cblist len == 1
// Warn because there is
// still a pending barrier
// trigger warning
WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
cpus_read_unlock();

                                                                // wait CPU1 comes online
                                                                // invoke barrier callback on
                                                                // CPU1 rdp's->cblist
                                                                wait_for_completion(&rcu_state.barrier_completion);
// deoffload CPU4
cpus_read_lock()
  rcu_barrier()
    mutex_lock(&rcu_state.barrier_mutex);
    // block on barrier_mutex
    // wait rcu_barrier() on
    // CPU3 to unlock barrier_mutex
    // but CPU3 unlock barrier_mutex
    // need to wait CPU1 comes online
    // when CPU1 going online will block on cpus_write_lock

The above scenario will not only trigger WARN_ON_ONCE(), but also
trigger deadlock, this commit therefore check rdp->cblist length
before invoke kthread_parkme(), and the kthread_parkme() is not
invoke until length reaches zero.

Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
---
 kernel/rcu/tree_nocb.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 8648233e1717..a2b0ebdefee3 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -893,6 +893,12 @@ static inline bool nocb_cb_wait_cond(struct rcu_data *rdp)
 	return !READ_ONCE(rdp->nocb_cb_sleep) || kthread_should_park();
 }
 
+static inline bool nocb_cblist_empty(struct rcu_data *rdp)
+{
+	return !(rcu_rdp_is_offloaded(rdp) &&
+		WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist)));
+}
+
 /*
  * Invoke any ready callbacks from the corresponding no-CBs CPU,
  * then, if there are no more, wait for more to appear.
@@ -907,7 +913,7 @@ static void nocb_cb_wait(struct rcu_data *rdp)
 
 	swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
 					    nocb_cb_wait_cond(rdp));
-	if (kthread_should_park()) {
+	if (kthread_should_park() && nocb_cblist_empty(rdp)) {
 		kthread_parkme();
 	} else if (READ_ONCE(rdp->nocb_cb_sleep)) {
 		WARN_ON(signal_pending(current));
-- 
2.17.1