Patchwork [v4] powerpc: kvm: fix rare but potential deadlock scene

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Submitter pingfan liu
Date Nov. 15, 2013, 8:35 a.m.
Message ID <1384504501-19348-1-git-send-email-pingfank@linux.vnet.ibm.com>
Download mbox | patch
Permalink /patch/291505/
State New
Headers show

Comments

pingfan liu - Nov. 15, 2013, 8:35 a.m.
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>
---
v4: remove the over-engineered part and keep it simple, also add some notes.
  
---
 arch/powerpc/kvm/book3s_64_mmu_hv.c | 6 +++++-
 arch/powerpc/kvm/book3s_hv_rm_mmu.c | 4 ++++
 2 files changed, 9 insertions(+), 1 deletion(-)
Paul Mackerras - Nov. 16, 2013, 6:55 a.m.
On Fri, Nov 15, 2013 at 04:35:00PM +0800, Liu Ping Fan 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@linux.vnet.ibm.com>

Reviewed-by: Paul Mackerras <paulus@samba.org>
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Alexander Graf - Nov. 18, 2013, 9:32 p.m.
On 16.11.2013, at 01:55, Paul Mackerras <paulus@samba.org> wrote:

> On Fri, Nov 15, 2013 at 04:35:00PM +0800, Liu Ping Fan 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@linux.vnet.ibm.com>
> 
> Reviewed-by: Paul Mackerras <paulus@samba.org>

Thanks, applied to kvm-ppc-queue.


Alex

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Alexander Graf - Nov. 18, 2013, 9:43 p.m.
On 18.11.2013, at 16:32, Alexander Graf <agraf@suse.de> wrote:

> 
> On 16.11.2013, at 01:55, Paul Mackerras <paulus@samba.org> wrote:
> 
>> On Fri, Nov 15, 2013 at 04:35:00PM +0800, Liu Ping Fan 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@linux.vnet.ibm.com>
>> 
>> Reviewed-by: Paul Mackerras <paulus@samba.org>
> 
> Thanks, applied to kvm-ppc-queue.

Actually, I've changed my mind and moved the patch to the for-3.13 branch instead. Please make sure to CC kvm@vger on all patches you submit though.


Alex

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Patch

diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index 842f081..abf81fe 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -473,11 +473,14 @@  static int kvmppc_mmu_book3s_64_hv_xlate(struct kvm_vcpu *vcpu, gva_t eaddr,
 		slb_v = vcpu->kvm->arch.vrma_slb_v;
 	}
 
+	preempt_disable();
 	/* Find the HPTE in the hash table */
 	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);
diff --git a/arch/powerpc/kvm/book3s_hv_rm_mmu.c b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
index 9c51544..ea17b30 100644
--- a/arch/powerpc/kvm/book3s_hv_rm_mmu.c
+++ b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
@@ -749,6 +749,10 @@  static int slb_base_page_shift[4] = {
 	20,	/* 1M, unsupported */
 };
 
+/* When called from virtmode, this func should be protected by
+ * preempt_disable(), otherwise, the holding of HPTE_V_HVLOCK
+ * can trigger deadlock issue.
+ */
 long kvmppc_hv_find_lock_hpte(struct kvm *kvm, gva_t eaddr, unsigned long slb_v,
 			      unsigned long valid)
 {