[PATCH 1/3] KVM: Show lockdep the kvm->mutex vs. kvm->srcu ordering rule

[David Woodhouse]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

Lockdep is learning to spot deadlocks with sleepable RCU vs. mutexes,
which can occur where one code path calls synchronize_scru() with a
mutex held, while another code path attempts to obtain the same mutex
while in a read-side section.

Since lockdep isn't very good at reading the English prose in
Documentation/virt/kvm/locking.rst, give it a demonstration by calling
synchronize_scru(&kvm->srcu) while holding kvm->lock in kvm_create_vm().
The cases where this happens naturally are relatively esoteric and may
not happen otherwise.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 virt/kvm/kvm_main.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 13e88297f999..285b3c5a6364 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1173,6 +1173,16 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
 	if (init_srcu_struct(&kvm->irq_srcu))
 		goto out_err_no_irq_srcu;
 
+#ifdef CONFIG_LOCKDEP
+	/*
+	 * Ensure lockdep knows that it's not permitted to lock kvm->lock
+	 * from a SRCU read section on kvm->srcu.
+	 */
+	mutex_lock(&kvm->lock);
+	synchronize_srcu(&kvm->srcu);
+	mutex_unlock(&kvm->lock);
+#endif
+
 	refcount_set(&kvm->users_count, 1);
 	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
 		for (j = 0; j < 2; j++) {
-- 
2.35.3