[PATCH v2] x86/xen: don't unbind uninitialized lock_kicker_irq

From: Brian Masney
Date: Fri Nov 06 2020 - 20:11:43 EST


When booting a hyperthreaded system with the kernel parameter
'mitigations=auto,nosmt', the following warning occurs:

WARNING: CPU: 0 PID: 1 at drivers/xen/events/events_base.c:1112 unbind_from_irqhandler+0x4e/0x60
...
Hardware name: Xen HVM domU, BIOS 4.2.amazon 08/24/2006
...
Call Trace:
xen_uninit_lock_cpu+0x28/0x62
xen_hvm_cpu_die+0x21/0x30
takedown_cpu+0x9c/0xe0
? trace_suspend_resume+0x60/0x60
cpuhp_invoke_callback+0x9a/0x530
_cpu_up+0x11a/0x130
cpu_up+0x7e/0xc0
bringup_nonboot_cpus+0x48/0x50
smp_init+0x26/0x79
kernel_init_freeable+0xea/0x229
? rest_init+0xaa/0xaa
kernel_init+0xa/0x106
ret_from_fork+0x35/0x40

The secondary CPUs are not activated with the nosmt mitigations and only
the primary thread on each CPU core is used. In this situation,
xen_hvm_smp_prepare_cpus(), and more importantly xen_init_lock_cpu(), is
not called, so the lock_kicker_irq is not initialized for the secondary
CPUs. Let's fix this by exiting early in xen_uninit_lock_cpu() if the
irq is not set to avoid the warning from above for each secondary CPU.

Signed-off-by: Brian Masney <bmasney@xxxxxxxxxx>
---
Changes since v1:
- Remove duplicate per_cpu() call and pass in irq variable.
- Changed subject from 'x86/xen: fix warning when running with nosmt
mitigations'
- Shorten code comment

arch/x86/xen/spinlock.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 799f4eba0a62..043c73dfd2c9 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -93,10 +93,20 @@ void xen_init_lock_cpu(int cpu)

void xen_uninit_lock_cpu(int cpu)
{
+ int irq;
+
if (!xen_pvspin)
return;

- unbind_from_irqhandler(per_cpu(lock_kicker_irq, cpu), NULL);
+ /*
+ * When booting the kernel with 'mitigations=auto,nosmt', the secondary
+ * CPUs are not activated, and lock_kicker_irq is not initialized.
+ */
+ irq = per_cpu(lock_kicker_irq, cpu);
+ if (irq == -1)
+ return;
+
+ unbind_from_irqhandler(irq, NULL);
per_cpu(lock_kicker_irq, cpu) = -1;
kfree(per_cpu(irq_name, cpu));
per_cpu(irq_name, cpu) = NULL;
--
2.26.2