[PATCH linux-next][RFC] powerpc: avoid lockdep when we are offline
From: Zhouyi Zhou
Date: Mon Sep 26 2022 - 21:48:47 EST
This is second version of my fix to PPC's "WARNING: suspicious RCU usage",
I improved my fix under Paul E. McKenney's guidance:
Link: https://lore.kernel.org/lkml/20220914021528.15946-1-zhouzhouyi@xxxxxxxxx/T/
During the cpu offlining, the sub functions of xive_teardown_cpu will
call __lock_acquire when CONFIG_LOCKDEP=y. The latter function will
travel RCU protected list, so "WARNING: suspicious RCU usage" will be
triggered.
Avoid lockdep when we are offline.
Signed-off-by: Zhouyi Zhou <zhouzhouyi@xxxxxxxxx>
---
Dear PPC and RCU developers
I found this bug when trying to do rcutorture tests in ppc VM of
Open Source Lab of Oregon State University
console.log report following bug:
[ 37.635545][ T0] WARNING: suspicious RCU usage^M
[ 37.636409][ T0] 6.0.0-rc4-next-20220907-dirty #8 Not tainted^M
[ 37.637575][ T0] -----------------------------^M
[ 37.638306][ T0] kernel/locking/lockdep.c:3723 RCU-list traversed in non-reader section!!^M
[ 37.639651][ T0] ^M
[ 37.639651][ T0] other info that might help us debug this:^M
[ 37.639651][ T0] ^M
[ 37.641381][ T0] ^M
[ 37.641381][ T0] RCU used illegally from offline CPU!^M
[ 37.641381][ T0] rcu_scheduler_active = 2, debug_locks = 1^M
[ 37.667170][ T0] no locks held by swapper/6/0.^M
[ 37.668328][ T0] ^M
[ 37.668328][ T0] stack backtrace:^M
[ 37.669995][ T0] CPU: 6 PID: 0 Comm: swapper/6 Not tainted 6.0.0-rc4-next-20220907-dirty #8^M
[ 37.672777][ T0] Call Trace:^M
[ 37.673729][ T0] [c000000004653920] [c00000000097f9b4] dump_stack_lvl+0x98/0xe0 (unreliable)^M
[ 37.678579][ T0] [c000000004653960] [c0000000001f2eb8] lockdep_rcu_suspicious+0x148/0x16c^M
[ 37.680425][ T0] [c0000000046539f0] [c0000000001ed9b4] __lock_acquire+0x10f4/0x26e0^M
[ 37.682450][ T0] [c000000004653b30] [c0000000001efc2c] lock_acquire+0x12c/0x420^M
[ 37.684113][ T0] [c000000004653c20] [c0000000010d704c] _raw_spin_lock_irqsave+0x6c/0xc0^M
[ 37.686154][ T0] [c000000004653c60] [c0000000000c7b4c] xive_spapr_put_ipi+0xcc/0x150^M
[ 37.687879][ T0] [c000000004653ca0] [c0000000010c72a8] xive_cleanup_cpu_ipi+0xc8/0xf0^M
[ 37.689856][ T0] [c000000004653cf0] [c0000000010c7370] xive_teardown_cpu+0xa0/0xf0^M
[ 37.691877][ T0] [c000000004653d30] [c0000000000fba5c] pseries_cpu_offline_self+0x5c/0x100^M
[ 37.693882][ T0] [c000000004653da0] [c00000000005d2c4] arch_cpu_idle_dead+0x44/0x60^M
[ 37.695739][ T0] [c000000004653dc0] [c0000000001c740c] do_idle+0x16c/0x3d0^M
[ 37.697536][ T0] [c000000004653e70] [c0000000001c7a1c] cpu_startup_entry+0x3c/0x40^M
[ 37.699694][ T0] [c000000004653ea0] [c00000000005ca20] start_secondary+0x6c0/0xb50^M
[ 37.701742][ T0] [c000000004653f90] [c00000000000d054] start_secondary_prolog+0x10/0x14^M
Tested on PPC VM of Open Source Lab of Oregon State University.
Test results show that although "WARNING: suspicious RCU usage" has gone,
and there are less "BUG: soft lockup" reports than the original kernel
(9 vs 13), which sounds good ;-)
But after my modification, results-rcutorture-kasan/SRCU-P/console.log.diags
shows a new warning:
[ 222.289242][ T110] WARNING: CPU: 6 PID: 110 at kernel/rcu/rcutorture.c:2806 rcu_torture_fwd_prog+0xc88/0xdd0
I guess above new warning also exits in original kernel, so I write a tiny test script as follows:
#!/bin/sh
COUNTER=0
while [ $COUNTER -lt 1000 ] ; do
qemu-system-ppc64 -nographic -smp cores=8,threads=1 -net none -M pseries -nodefaults -device spapr-vscsi -serial file:/tmp/console.log -m 2G -kernel /tmp/vmlinux -append "debug_boot_weak_hash panic=-1 console=ttyS0 rcupdate.rcu_cpu_stall_suppress_at_boot=1 torture.disable_onoff_at_boot rcupdate.rcu_task_stall_timeout=30000 rcutorture.torture_type=srcud rcupdate.rcu_self_test=1 rcutorture.fwd_progress=3 srcutree.big_cpu_lim=5 rcutorture.onoff_interval=1000 rcutorture.onoff_holdoff=30 rcutorture.n_barrier_cbs=4 rcutorture.stat_interval=15 rcutorture.shutdown_secs=420 rcutorture.test_no_idle_hz=1 rcutorture.verbose=1"&
qemu_pid=$!
cd ~/next1/linux-next
make clean
#I use "make vmlinux -j 8" to create heavy background jitter
make vmlinux -j 8 > /dev/null 2>&1
make_pid=$!
wait $qemu_pid
kill $qemu_pid
kill $make_id
if grep -q WARN /tmp/console.log;
then
echo $COUNTER > /tmp/counter
exit
fi
COUNTER=$(($COUNTER+1))
done
Above test shows that original kernel also warn about
"WARNING: CPU: 6 PID: 110 at kernel/rcu/rcutorture.c:2806 rcu_torture_fwd_prog+0xc88/0xdd0"
But I am not very sure about my results, so I still add a [RFC] to my subject line.
Thank all of you for your guidance and encouragement ;-)
Cheers
Zhouyi
--
arch/powerpc/platforms/pseries/hotplug-cpu.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
index e0a7ac5db15d..e47098f00da1 100644
--- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
+++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
@@ -64,10 +64,15 @@ static void pseries_cpu_offline_self(void)
local_irq_disable();
idle_task_exit();
+ /* prevent lockdep code from traveling RCU protected list
+ * when we are offline.
+ */
+ lockdep_off();
if (xive_enabled())
xive_teardown_cpu();
else
xics_teardown_cpu();
+ lockdep_on();
unregister_slb_shadow(hwcpu);
rtas_stop_self();
--
2.25.1