Re: KVM: x86: fix deadlock in clock-in-progress request handling

[Gleb Natapov <gleb@redhat.com>]

On Mon, Mar 18, 2013 at 01:54:32PM -0300, Marcelo Tosatti wrote:
> 
> There is a deadlock in pvclock handling:
> 
> cpu0:                                               cpu1:
> kvm_gen_update_masterclock()
>                                               kvm_guest_time_update()
>  spin_lock(pvclock_gtod_sync_lock)
>                                                local_irq_save(flags)
>                                               
> spin_lock(pvclock_gtod_sync_lock)
> 
>  kvm_make_mclock_inprogress_request(kvm)
>   make_all_cpus_request()
>    smp_call_function_many()
> 
> Now if smp_call_function_many() called by cpu0 tries to call function on
> cpu1 there will be a deadlock.
> 
> Fix by moving pvclock_gtod_sync_lock protected section outside irq
> disabled section.
> 
> Analyzed by Gleb Natapov <gleb@redhat.com>
> Reported-and-Tested-by: Yongjie Ren <yongjie.ren@intel.com>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
Acked-by: Gleb Natapov <gleb@redhat.com>

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index f71500a..f7c850b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1416,15 +1416,6 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
>  	kernel_ns = 0;
>  	host_tsc = 0;
>  
> -	/* Keep irq disabled to prevent changes to the clock */
> -	local_irq_save(flags);
> -	this_tsc_khz = __get_cpu_var(cpu_tsc_khz);
> -	if (unlikely(this_tsc_khz == 0)) {
> -		local_irq_restore(flags);
> -		kvm_make_request(KVM_REQ_CLOCK_UPDATE, v);
> -		return 1;
> -	}
> -
>  	/*
>  	 * If the host uses TSC clock, then passthrough TSC as stable
>  	 * to the guest.
> @@ -1436,6 +1427,15 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
>  		kernel_ns = ka->master_kernel_ns;
>  	}
>  	spin_unlock(&ka->pvclock_gtod_sync_lock);
> +
> +	/* Keep irq disabled to prevent changes to the clock */
> +	local_irq_save(flags);
> +	this_tsc_khz = __get_cpu_var(cpu_tsc_khz);
> +	if (unlikely(this_tsc_khz == 0)) {
> +		local_irq_restore(flags);
> +		kvm_make_request(KVM_REQ_CLOCK_UPDATE, v);
> +		return 1;
> +	}
>  	if (!use_master_clock) {
>  		host_tsc = native_read_tsc();
>  		kernel_ns = get_kernel_ns();

--
			Gleb.