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Subject[PATCH RT v3 4/5] rcu: Disable use_softirq on PREEMPT_RT
Date
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>
---
The prohibition on use_softirq should be able to be dropped once RT gets
the latest RCU code, but the question of what use_softirq should default
to on PREEMPT_RT remains.

v3: Use IS_ENABLED
---
kernel/rcu/tree.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index fc8b00c61b32..ee0a5ec2c30f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -98,9 +98,14 @@ 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. */
-static bool use_softirq = 1;
+/*
+ * By default, use RCU_SOFTIRQ instead of rcuc kthreads.
+ * But, avoid RCU_SOFTIRQ on PREEMPT_RT due to pi/rq deadlocks.
+ */
+static bool use_softirq = !IS_ENABLED(CONFIG_PREEMPT_RT_FULL);
+#ifdef CONFIG_PREEMPT_RT_FULL
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
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