On 6/8/24 02:06, Sean Christopherson wrote:
Use a dedicated mutex to guard kvm_usage_count to fix a potential deadlock
on x86 due to a chain of locks and SRCU synchronizations. Translating the
below lockdep splat, CPU1 #6 will wait on CPU0 #1, CPU0 #8 will wait on
CPU2 #3, and CPU2 #7 will wait on CPU1 #4 (if there's a writer, due to the
fairness of r/w semaphores).
CPU0 CPU1 CPU2
1 lock(&kvm->slots_lock);
2 lock(&vcpu->mutex);
3 lock(&kvm->srcu);
4 lock(cpu_hotplug_lock);
5 lock(kvm_lock);
6 lock(&kvm->slots_lock);
7 lock(cpu_hotplug_lock);
8 sync(&kvm->srcu);
Note, there are likely more potential deadlocks in KVM x86, e.g. the same
pattern of taking cpu_hotplug_lock outside of kvm_lock likely exists with
__kvmclock_cpufreq_notifier()
Offhand I couldn't see any places where {,__}cpufreq_driver_target() is
called within cpus_read_lock(). I didn't look too closely though.
+``kvm_usage_count``
+^^^^^^^^^^^^^^^^^^^
``kvm_usage_lock``
Paolo
+
+:Type: mutex
+:Arch: any
+:Protects: - kvm_usage_count
- hardware virtualization enable/disable
:Comment: KVM also disables CPU hotplug via cpus_read_lock() during
enable/disable.
@@ -290,11 +296,12 @@ time it will be set using the Dirty tracking mechanism described above.
wakeup.
``vendor_module_lock``
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^^^^^^^^^^
:Type: mutex
:Arch: x86
:Protects: loading a vendor module (kvm_amd or kvm_intel)
-:Comment: Exists because using kvm_lock leads to deadlock. cpu_hotplug_lock is
- taken outside of kvm_lock, e.g. in KVM's CPU online/offline callbacks, and
- many operations need to take cpu_hotplug_lock when loading a vendor module,
- e.g. updating static calls.
+:Comment: Exists because using kvm_lock leads to deadlock. kvm_lock is taken
+ in notifiers, e.g. __kvmclock_cpufreq_notifier(), that may be invoked while
+ cpu_hotplug_lock is held, e.g. from cpufreq_boost_trigger_state(), and many
+ operations need to take cpu_hotplug_lock when loading a vendor module, e.g.
+ updating static calls.
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 4965196cad58..d9b0579d3eea 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -5499,6 +5499,7 @@ __visible bool kvm_rebooting;
EXPORT_SYMBOL_GPL(kvm_rebooting);
static DEFINE_PER_CPU(bool, hardware_enabled);
+static DEFINE_MUTEX(kvm_usage_lock);
static int kvm_usage_count;
static int __hardware_enable_nolock(void)
@@ -5531,10 +5532,10 @@ static int kvm_online_cpu(unsigned int cpu)
* be enabled. Otherwise running VMs would encounter unrecoverable
* errors when scheduled to this CPU.
*/
- mutex_lock(&kvm_lock);
+ mutex_lock(&kvm_usage_lock);
if (kvm_usage_count)
ret = __hardware_enable_nolock();
- mutex_unlock(&kvm_lock);
+ mutex_unlock(&kvm_usage_lock);
return ret;
}
@@ -5554,10 +5555,10 @@ static void hardware_disable_nolock(void *junk)
static int kvm_offline_cpu(unsigned int cpu)
{
- mutex_lock(&kvm_lock);
+ mutex_lock(&kvm_usage_lock);
if (kvm_usage_count)
hardware_disable_nolock(NULL);
- mutex_unlock(&kvm_lock);
+ mutex_unlock(&kvm_usage_lock);
return 0;
}
@@ -5573,9 +5574,9 @@ static void hardware_disable_all_nolock(void)
static void hardware_disable_all(void)
{
cpus_read_lock();
- mutex_lock(&kvm_lock);
+ mutex_lock(&kvm_usage_lock);
hardware_disable_all_nolock();
- mutex_unlock(&kvm_lock);
+ mutex_unlock(&kvm_usage_lock);
cpus_read_unlock();
}
@@ -5606,7 +5607,7 @@ static int hardware_enable_all(void)
* enable hardware multiple times.
*/
cpus_read_lock();
- mutex_lock(&kvm_lock);
+ mutex_lock(&kvm_usage_lock);
r = 0;
@@ -5620,7 +5621,7 @@ static int hardware_enable_all(void)
}
}
- mutex_unlock(&kvm_lock);
+ mutex_unlock(&kvm_usage_lock);
cpus_read_unlock();
return r;
@@ -5648,13 +5649,13 @@ static int kvm_suspend(void)
{
/*
* Secondary CPUs and CPU hotplug are disabled across the suspend/resume
- * callbacks, i.e. no need to acquire kvm_lock to ensure the usage count
- * is stable. Assert that kvm_lock is not held to ensure the system
- * isn't suspended while KVM is enabling hardware. Hardware enabling
- * can be preempted, but the task cannot be frozen until it has dropped
- * all locks (userspace tasks are frozen via a fake signal).
+ * callbacks, i.e. no need to acquire kvm_usage_lock to ensure the usage
+ * count is stable. Assert that kvm_usage_lock is not held to ensure
+ * the system isn't suspended while KVM is enabling hardware. Hardware
+ * enabling can be preempted, but the task cannot be frozen until it has
+ * dropped all locks (userspace tasks are frozen via a fake signal).
*/
- lockdep_assert_not_held(&kvm_lock);
+ lockdep_assert_not_held(&kvm_usage_lock);
lockdep_assert_irqs_disabled();
if (kvm_usage_count)
@@ -5664,7 +5665,7 @@ static int kvm_suspend(void)
static void kvm_resume(void)
{
- lockdep_assert_not_held(&kvm_lock);
+ lockdep_assert_not_held(&kvm_usage_lock);
lockdep_assert_irqs_disabled();
if (kvm_usage_count)
