[PATCH RT v2 3/3] rcu: Disable use_softirq on PREEMPT_RT

[Scott Wood <swood@redhat.com>]

Besides restoring behavior that used to be default on RT, this avoids
a deadlock on scheduler locks:

[  136.894657] 039: ============================================
[  136.900401] 039: WARNING: possible recursive locking detected
[  136.906146] 039: 5.2.9-rt3.dbg+ #174 Tainted: G            E
[  136.912152] 039: --------------------------------------------
[  136.917896] 039: rcu_torture_rea/13474 is trying to acquire lock:
[  136.923990] 039: 000000005f25146d
[  136.927310] 039:  (
[  136.929414] 039: &p->pi_lock
[  136.932303] 039: ){-...}
[  136.934840] 039: , at: try_to_wake_up+0x39/0x920
[  136.939461] 039:
but task is already holding lock:
[  136.944425] 039: 000000005f25146d
[  136.947745] 039:  (
[  136.949852] 039: &p->pi_lock
[  136.952738] 039: ){-...}
[  136.955274] 039: , at: try_to_wake_up+0x39/0x920
[  136.959895] 039:
other info that might help us debug this:
[  136.965555] 039:  Possible unsafe locking scenario:

[  136.970608] 039:        CPU0
[  136.973493] 039:        ----
[  136.976378] 039:   lock(
[  136.978918] 039: &p->pi_lock
[  136.981806] 039: );
[  136.983911] 039:   lock(
[  136.986451] 039: &p->pi_lock
[  136.989336] 039: );
[  136.991444] 039:
 *** DEADLOCK ***

[  136.995194] 039:  May be due to missing lock nesting notation

[  137.001115] 039: 3 locks held by rcu_torture_rea/13474:
[  137.006341] 039:  #0:
[  137.008707] 039: 000000005f25146d
[  137.012024] 039:  (
[  137.014131] 039: &p->pi_lock
[  137.017015] 039: ){-...}
[  137.019558] 039: , at: try_to_wake_up+0x39/0x920
[  137.024175] 039:  #1:
[  137.026540] 039: 0000000011c8e51d
[  137.029859] 039:  (
[  137.031966] 039: &rq->lock
[  137.034679] 039: ){-...}
[  137.037217] 039: , at: try_to_wake_up+0x241/0x920
[  137.041924] 039:  #2:
[  137.044291] 039: 00000000098649b9
[  137.047610] 039:  (
[  137.049714] 039: rcu_read_lock
[  137.052774] 039: ){....}
[  137.055314] 039: , at: cpuacct_charge+0x33/0x1e0
[  137.059934] 039:
stack backtrace:
[  137.063425] 039: CPU: 39 PID: 13474 Comm: rcu_torture_rea Kdump: loaded Tainted: G            E     5.2.9-rt3.dbg+ #174
[  137.074197] 039: Hardware name: Intel Corporation S2600BT/S2600BT, BIOS SE5C620.86B.01.00.0763.022420181017 02/24/2018
[  137.084886] 039: Call Trace:
[  137.087773] 039:  <IRQ>
[  137.090226] 039:  dump_stack+0x5e/0x8b
[  137.093997] 039:  __lock_acquire+0x725/0x1100
[  137.098358] 039:  lock_acquire+0xc0/0x240
[  137.102374] 039:  ? try_to_wake_up+0x39/0x920
[  137.106737] 039:  _raw_spin_lock_irqsave+0x47/0x90
[  137.111534] 039:  ? try_to_wake_up+0x39/0x920
[  137.115910] 039:  try_to_wake_up+0x39/0x920
[  137.120098] 039:  rcu_read_unlock_special+0x65/0xb0
[  137.124977] 039:  __rcu_read_unlock+0x5d/0x70
[  137.129337] 039:  cpuacct_charge+0xd9/0x1e0
[  137.133522] 039:  ? cpuacct_charge+0x33/0x1e0
[  137.137880] 039:  update_curr+0x14b/0x420
[  137.141894] 039:  enqueue_entity+0x42/0x370
[  137.146080] 039:  enqueue_task_fair+0xa9/0x490
[  137.150528] 039:  activate_task+0x5a/0xf0
[  137.154539] 039:  ttwu_do_activate+0x4e/0x90
[  137.158813] 039:  try_to_wake_up+0x277/0x920
[  137.163086] 039:  irq_exit+0xb6/0xf0
[  137.166661] 039:  smp_apic_timer_interrupt+0xe3/0x3a0
[  137.171714] 039:  apic_timer_interrupt+0xf/0x20
[  137.176249] 039:  </IRQ>
[  137.178785] 039: RIP: 0010:__schedule+0x0/0x8e0
[  137.183319] 039: Code: 00 02 48 89 43 20 e8 0f 5a 00 00 48 8d 7b 28 e8 86 f2 fd ff 31 c0 5b 5d 41 5c c3 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 <55> 48 89 e5 41 57 41 56 49 c7 c6 c0 ca 1e 00 41 55 41 89 fd 41 54
[  137.202498] 039: RSP: 0018:ffffc9005835fbc0 EFLAGS: 00000246
[  137.208158] 039:  ORIG_RAX: ffffffffffffff13
[  137.212428] 039: RAX: 0000000000000000 RBX: ffff8897c3e1bb00 RCX: 0000000000000001
[  137.219994] 039: RDX: 0000000080004008 RSI: 0000000000000006 RDI: 0000000000000001
[  137.227560] 039: RBP: ffff8897c3e1bb00 R08: 0000000000000000 R09: 0000000000000000
[  137.235126] 039: R10: 0000000000000001 R11: 0000000000000001 R12: ffffffff81001fd1
[  137.242694] 039: R13: 0000000000000044 R14: 0000000000000000 R15: ffffc9005835fcac
[  137.250259] 039:  ? ___preempt_schedule+0x16/0x18
[  137.254969] 039:  preempt_schedule_common+0x32/0x80
[  137.259846] 039:  ___preempt_schedule+0x16/0x18
[  137.264379] 039:  rcutorture_one_extend+0x33a/0x510 [rcutorture]
[  137.270397] 039:  rcu_torture_one_read+0x18c/0x450 [rcutorture]
[  137.276334] 039:  rcu_torture_reader+0xac/0x1f0 [rcutorture]
[  137.281998] 039:  ? rcu_torture_reader+0x1f0/0x1f0 [rcutorture]
[  137.287920] 039:  kthread+0x106/0x140
[  137.291591] 039:  ? rcu_torture_one_read+0x450/0x450 [rcutorture]
[  137.297681] 039:  ? kthread_bind+0x10/0x10
[  137.301783] 039:  ret_from_fork+0x3a/0x50

Signed-off-by: Scott Wood <swood@redhat.com>
---
I think the prohibition on use_softirq can be dropped once RT gets the
latest RCU code, but the question of what use_softirq should default
to on PREEMPT_RT remains.
---
 kernel/rcu/tree.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index fc8b00c61b32..12036d33e2f9 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -98,9 +98,16 @@ struct rcu_state rcu_state = {
 /* Dump rcu_node combining tree at boot to verify correct setup. */
 static bool dump_tree;
 module_param(dump_tree, bool, 0444);
-/* By default, use RCU_SOFTIRQ instead of rcuc kthreads. */
+/*
+ * By default, use RCU_SOFTIRQ instead of rcuc kthreads.
+ * But, avoid RCU_SOFTIRQ on PREEMPT_RT due to pi/rq deadlocks.
+ */
+#ifdef CONFIG_PREEMPT_RT_FULL
+static bool use_softirq;
+#else
 static bool use_softirq = 1;
 module_param(use_softirq, bool, 0444);
+#endif
 /* Control rcu_node-tree auto-balancing at boot time. */
 static bool rcu_fanout_exact;
 module_param(rcu_fanout_exact, bool, 0444);
-- 
1.8.3.1