On 13.09.2017 14:17, Christian Borntraeger wrote:
> commit 3898da947bba ("KVM: avoid using rcu_dereference_protected") can
> trigger the following lockdep/rcu splat if the VM_CREATE ioctl fails,
> for example if kvm_arch_init_vm fails:
>
> WARNING: suspicious RCU usage
> 4.13.0+ #105 Not tainted
> -----------------------------
> ./include/linux/kvm_host.h:481 suspicious rcu_dereference_check() usage!
>
> other info that might help us debug this:
>
> rcu_scheduler_active = 2, debug_locks = 1
> no locks held by qemu-system-s39/79.
> stack backtrace:
> CPU: 0 PID: 79 Comm: qemu-system-s39 Not tainted 4.13.0+ #105
> Hardware name: IBM 2964 NC9 704 (KVM/Linux)
> Call Trace:
> ([<00000000001140b2>] show_stack+0xea/0xf0)
> [<00000000008a68a4>] dump_stack+0x94/0xd8
> [<0000000000134c12>] kvm_dev_ioctl+0x372/0x7a0
> [<000000000038f940>] do_vfs_ioctl+0xa8/0x6c8
> [<0000000000390004>] SyS_ioctl+0xa4/0xb8
> [<00000000008c7a8c>] system_call+0xc4/0x27c
> no locks held by qemu-system-s39/79.
>
> We have to reset the just created users_count back to 0 to
> tell the check to not trigger.
>
> Reported-by: Stefan Haberland <sth@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Fixes: 3898da947bba ("KVM: avoid using rcu_dereference_protected")
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
> virt/kvm/kvm_main.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 1b3fa3f..1205d20 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -716,6 +716,7 @@ static struct kvm *kvm_create_vm(unsigned long type)
> out_err_no_srcu:
> hardware_disable_all();
> out_err_no_disable:
> + refcount_set(&kvm->users_count, 0);
> for (i = 0; i < KVM_NR_BUSES; i++)
> kfree(kvm_get_bus(kvm, i));
> for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
>
Makes sense to me.
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
--
Thanks,
David
