[PATCH v2 1/2] powerpc/kvm: fix rare but potential deadlock scene

[Alexander Graf]

On 07.11.2013, at 07:22, Liu Ping Fan <kernelfans at gmail.com> wrote:

> Since kvmppc_hv_find_lock_hpte() is called from both virtmode and
> realmode, so it can trigger the deadlock.
> 
> Suppose the following scene:
> 
> Two physical cpuM, cpuN, two VM instances A, B, each VM has a group of vcpus.
> 
> If on cpuM, vcpu_A_1 holds bitlock X (HPTE_V_HVLOCK), then is switched out,
> and on cpuN, vcpu_A_2 try to lock X in realmode, then cpuN will be caught in
> realmode for a long time.
> 
> What makes things even worse if the following happens,
>  On cpuM, bitlockX is hold, on cpuN, Y is hold.
>  vcpu_B_2 try to lock Y on cpuM in realmode
>  vcpu_A_2 try to lock X on cpuN in realmode
> 
> Oops! deadlock happens
> 
> Signed-off-by: Liu Ping Fan <pingfank at linux.vnet.ibm.com>

Very nice catch :).

I think it makes a lot of sense to document the fact that kvmppc_hv_find_lock_hpte() should be called with preemption disabled in a comment above the function, so that next time when someone potentially calls it, he knows that he needs to put preempt_disable() around it.

Thanks a lot for finding this pretty subtle issue. May I ask how you got there? Did you actually see systems deadlock because of this?


Alex

> ---
> arch/powerpc/kvm/book3s_64_mmu_hv.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> index 043eec8..dbc1478 100644
> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> @@ -474,10 +474,13 @@ static int kvmppc_mmu_book3s_64_hv_xlate(struct kvm_vcpu *vcpu, gva_t eaddr,
> 	}
> 
> 	/* Find the HPTE in the hash table */
> +	preempt_disable();
> 	index = kvmppc_hv_find_lock_hpte(kvm, eaddr, slb_v,
> 					 HPTE_V_VALID | HPTE_V_ABSENT);
> -	if (index < 0)
> +	if (index < 0) {
> +		preempt_enable();
> 		return -ENOENT;
> +	}
> 	hptep = (unsigned long *)(kvm->arch.hpt_virt + (index << 4));
> 	v = hptep[0] & ~HPTE_V_HVLOCK;
> 	gr = kvm->arch.revmap[index].guest_rpte;
> @@ -485,6 +488,7 @@ static int kvmppc_mmu_book3s_64_hv_xlate(struct kvm_vcpu *vcpu, gva_t eaddr,
> 	/* Unlock the HPTE */
> 	asm volatile("lwsync" : : : "memory");
> 	hptep[0] = v;
> +	preempt_enable();
> 
> 	gpte->eaddr = eaddr;
> 	gpte->vpage = ((v & HPTE_V_AVPN) << 4) | ((eaddr >> 12) & 0xfff);
> -- 
> 1.8.1.4
>