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KVM: avoid using rcu_dereference_protected

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During teardown, accesses to memslots and buses are using
rcu_dereference_protected with an always-true condition because
these accesses are done outside the usual mutexes.  This
is because the last reference is gone and there cannot be any
concurrent modifications, but rcu_dereference_protected is
ugly and unobvious.

Instead, check the refcount in kvm_get_bus and __kvm_memslots.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
This commit is contained in:
Paolo Bonzini 2017-08-02 17:55:54 +02:00 committed by Radim Krčmář
parent ebd28fcb55
commit 3898da947b
2 changed files with 8 additions and 9 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

6
include/linux/kvm_host.h
View file

@ -477,7 +477,8 @@ struct kvm {
static inline struct kvm_io_bus *kvm_get_bus(struct kvm *kvm, enum kvm_bus idx)
{
return srcu_dereference_check(kvm->buses[idx], &kvm->srcu,
lockdep_is_held(&kvm->slots_lock));
lockdep_is_held(&kvm->slots_lock) ||
!refcount_read(&kvm->users_count));
}
static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
@ -570,7 +571,8 @@ void kvm_put_kvm(struct kvm *kvm);
static inline struct kvm_memslots *__kvm_memslots(struct kvm *kvm, int as_id)
{
return srcu_dereference_check(kvm->memslots[as_id], &kvm->srcu,
lockdep_is_held(&kvm->slots_lock));
lockdep_is_held(&kvm->slots_lock) ||
!refcount_read(&kvm->users_count));
}
static inline struct kvm_memslots *kvm_memslots(struct kvm *kvm)

11
virt/kvm/kvm_main.c
View file

@ -717,10 +717,9 @@ static struct kvm *kvm_create_vm(unsigned long type)
hardware_disable_all();
out_err_no_disable:
for (i = 0; i < KVM_NR_BUSES; i++)
kfree(rcu_access_pointer(kvm->buses[i]));
kfree(kvm_get_bus(kvm, i));
for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
kvm_free_memslots(kvm,
rcu_dereference_protected(kvm->memslots[i], 1));
kvm_free_memslots(kvm, __kvm_memslots(kvm, i));
kvm_arch_free_vm(kvm);
mmdrop(current->mm);
return ERR_PTR(r);
@ -754,9 +753,8 @@ static void kvm_destroy_vm(struct kvm *kvm)
spin_unlock(&kvm_lock);
kvm_free_irq_routing(kvm);
for (i = 0; i < KVM_NR_BUSES; i++) {
struct kvm_io_bus *bus;
struct kvm_io_bus *bus = kvm_get_bus(kvm, i);
bus = rcu_dereference_protected(kvm->buses[i], 1);
if (bus)
kvm_io_bus_destroy(bus);
kvm->buses[i] = NULL;
@ -770,8 +768,7 @@ static void kvm_destroy_vm(struct kvm *kvm)
kvm_arch_destroy_vm(kvm);
kvm_destroy_devices(kvm);
for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
kvm_free_memslots(kvm,
rcu_dereference_protected(kvm->memslots[i], 1));
kvm_free_memslots(kvm, __kvm_memslots(kvm, i));
cleanup_srcu_struct(&kvm->irq_srcu);
cleanup_srcu_struct(&kvm->srcu);
kvm_arch_free_vm(kvm);