[PATCH 2/3] powerpc/kvm: fix rare but potential deadlock scene

[Liu Ping Fan <kernelfans@gmail.com>]

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@linux.vnet.ibm.com>
---
 arch/powerpc/kvm/book3s_64_mmu_hv.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index 043eec8..28160ac 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -474,8 +474,10 @@ 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);
+	preempt_enable();
 	if (index < 0)
 		return -ENOENT;
 	hptep = (unsigned long *)(kvm->arch.hpt_virt + (index << 4));
-- 
1.8.1.4