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From:   Zhouyi Zhou <zhouzhouyi@gmail.com>
To:     mpe@ellerman.id.au, benh@kernel.crashing.org, paulus@samba.org,
        npiggin@gmail.com, ldufour@linux.ibm.com, pmladek@suse.com,
        john.ogness@linutronix.de, Julia.Lawall@inria.fr,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        lance@osuosl.org, paulmck@kernel.org, rcu@vger.kernel.org
Cc:     Zhouyi Zhou <zhouzhouyi@gmail.com>
Subject: [PATCH linux-next] powerpc: use raw_smp_processor_id in arch_touch_nmi_watchdog
Date:   Thu, 14 Jul 2022 09:31:31 +0800
Message-Id: <20220714013131.12648-1-zhouzhouyi@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
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use raw_smp_processor_id() in arch_touch_nmi_watchdog
because when called from watchdog, the cpu is preemptible.

Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
---
Dear PPC developers

I found this bug when trying to do rcutorture tests in ppc VM of
Open Source Lab of Oregon State University.

qemu-system-ppc64  -nographic -smp cores=4,threads=1 -net none  -M pseries -nodefaults -device spapr-vscsi -serial file:/tmp/console.log -m 2G -kernel /home/ubuntu/linux-next/tools/testing/selftests/rcutorture/res/2022.07.08-22.36.11-torture/results-rcuscale-kvfree/TREE/vmlinux -append "debug_boot_weak_hash panic=-1 console=ttyS0 rcuscale.kfree_rcu_test=1 rcuscale.kfree_nthreads=16 rcuscale.holdoff=20 rcuscale.kfree_loops=10000 torture.disable_onoff_at_boot rcuscale.shutdown=1 rcuscale.verbose=0"

tail /tmp/console.log
[ 1232.433552][   T41] BUG: using smp_processor_id() in preemptible [00000000] code: khungtaskd/41
[ 1232.439751][   T41] caller is arch_touch_nmi_watchdog+0x34/0xd0
[ 1232.440934][   T41] CPU: 3 PID: 41 Comm: khungtaskd Not tainted 5.19.0-rc5-next-20220708-dirty #106
[ 1232.442684][   T41] Call Trace:
[ 1232.443343][   T41] [c0000000029cbbb0] [c0000000006df360] dump_stack_lvl+0x74/0xa8 (unreliable)
[ 1232.445237][   T41] [c0000000029cbbf0] [c000000000d04f30] check_preemption_disabled+0x150/0x160
[ 1232.446926][   T41] [c0000000029cbc80] [c000000000035584] arch_touch_nmi_watchdog+0x34/0xd0
[ 1232.448532][   T41] [c0000000029cbcb0] [c0000000002068ac] watchdog+0x40c/0x5b0
[ 1232.451449][   T41] [c0000000029cbdc0] [c000000000139df4] kthread+0x144/0x170
[ 1232.452896][   T41] [c0000000029cbe10] [c00000000000cd54] ret_from_kernel_thread+0x5c/0x64

After this fix, "BUG: using smp_processor_id() in preemptible [00000000] code: khungtaskd/41" does not
appear again.

I also examined other places in watchdog.c where smp_processor_id() are used, but they are well protected by preempt
disable.

Kind Regards
Zhouyi
--
 arch/powerpc/kernel/watchdog.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c
index 7d28b9553654..ab6b84e00311 100644
--- a/arch/powerpc/kernel/watchdog.c
+++ b/arch/powerpc/kernel/watchdog.c
@@ -450,7 +450,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 void arch_touch_nmi_watchdog(void)
 {
 	unsigned long ticks = tb_ticks_per_usec * wd_timer_period_ms * 1000;
-	int cpu = smp_processor_id();
+	int cpu = raw_smp_processor_id();
 	u64 tb;
 
 	if (!cpumask_test_cpu(cpu, &watchdog_cpumask))
-- 
2.25.1