Re: [PATCH v4] KVM: x86: fix access memslots w/o hold srcu read lock

[Tang Chen <tangchen@cn.fujitsu.com>]

Thanks for the sharing. Will do more tests. :)

On 11/14/2014 07:39 AM, Wanpeng Li wrote:
> Hi Tang,
> On Tue, Nov 11, 2014 at 01:35:29PM +0800, Tang Chen wrote:
>> Hi Wanpeng,
>>
> Sorry for the late.
>
>> I think I have totally missed this thread.
>> I opened lockdep and RCU debug, and tried on 3.18-rc1. But I didn't
>> get the warning.
> I also opened lockdep and RCU debug, and tried 3.18.0-rc2 on a Ivy
> bridge, the warning will be triggered after run qemu immediately. There
> is no need to try any hotplug related stuff.
>
> In addition, Paolo's patch is merged upstream to fix this.
>
> commit a73896cb5bbdce672945745db8224352a689f580
> Author: Paolo Bonzini <pbonzini@redhat.com>
> Date:   Sun Nov 2 07:54:30 2014 +0100
>
> KVM: vmx: defer load of APIC access page address during reset
>
> Regards,
> Wanpeng Li
>
>> My steps are:
>>
>> 1. Use numactl to bind a qemu process to node1.
>> 2. Offline all node1 memory. And the qemu process is still running.
>>
>> Would you please tell me how did you reproduce it ?
>>
>> Thanks.
>>
>> On 11/02/2014 03:07 PM, Wanpeng Li wrote:
>>> The srcu read lock must be held while accessing memslots (e.g.
>>> when using gfn_to_* functions), however, commit c24ae0dcd3e8
>>> ("kvm: x86: Unpin and remove kvm_arch->apic_access_page") call
>>> gfn_to_page() in kvm_vcpu_reload_apic_access_page() w/o hold it in
>>> vmx_vcpu_reset() path which leads to suspicious rcu_dereference_check()
>>> usage warning. This patch fix it by holding srcu read lock in all
>>> kvm_vcpu_reset() call path.
>>>
>>>
>>> [ INFO: suspicious RCU usage. ]
>>> 3.18.0-rc2-test2+ #70 Not tainted
>>> -------------------------------
>>> include/linux/kvm_host.h:474 suspicious rcu_dereference_check() usage!
>>>
>>> other info that might help us debug this:
>>>
>>> rcu_scheduler_active = 1, debug_locks = 0
>>> 1 lock held by qemu-system-x86/2371:
>>>   #0:  (&vcpu->mutex){+.+...}, at: [<ffffffffa037d800>] vcpu_load+0x20/0xd0 [kvm]
>>>
>>> stack backtrace:
>>> CPU: 4 PID: 2371 Comm: qemu-system-x86 Not tainted 3.18.0-rc2-test2+ #70
>>> Hardware name: Dell Inc. OptiPlex 9010/0M9KCM, BIOS A12 01/10/2013
>>>   0000000000000001 ffff880209983ca8 ffffffff816f514f 0000000000000000
>>>   ffff8802099b8990 ffff880209983cd8 ffffffff810bd687 00000000000fee00
>>>   ffff880208a2c000 ffff880208a10000 ffff88020ef50040 ffff880209983d08
>>> Call Trace:
>>>   [<ffffffff816f514f>] dump_stack+0x4e/0x71
>>>   [<ffffffff810bd687>] lockdep_rcu_suspicious+0xe7/0x120
>>>   [<ffffffffa037d055>] gfn_to_memslot+0xd5/0xe0 [kvm]
>>>   [<ffffffffa03807d3>] __gfn_to_pfn+0x33/0x60 [kvm]
>>>   [<ffffffffa0380885>] gfn_to_page+0x25/0x90 [kvm]
>>>   [<ffffffffa038aeec>] kvm_vcpu_reload_apic_access_page+0x3c/0x80 [kvm]
>>>   [<ffffffffa08f0a9c>] vmx_vcpu_reset+0x20c/0x460 [kvm_intel]
>>>   [<ffffffffa039ab8e>] kvm_vcpu_reset+0x15e/0x1b0 [kvm]
>>>   [<ffffffffa039ac0c>] kvm_arch_vcpu_setup+0x2c/0x50 [kvm]
>>>   [<ffffffffa037f7e0>] kvm_vm_ioctl+0x1d0/0x780 [kvm]
>>>   [<ffffffff810bc664>] ? __lock_is_held+0x54/0x80
>>>   [<ffffffff812231f0>] do_vfs_ioctl+0x300/0x520
>>>   [<ffffffff8122ee45>] ? __fget+0x5/0x250
>>>   [<ffffffff8122f0fa>] ? __fget_light+0x2a/0xe0
>>>   [<ffffffff81223491>] SyS_ioctl+0x81/0xa0
>>>   [<ffffffff816fed6d>] system_call_fastpath+0x16/0x1b
>>>
>>> Reported-by: Takashi Iwai <tiwai@suse.de>
>>> Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
>>> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
>>> Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
>>> ---
>>> v3 -> v4:
>>>   * bypass the problem altoghter by kvm_make_request
>>> v2 -> v3:
>>>   * take care all vmx_vcpu_reset call path
>>> v1 -> v2:
>>>   * just fix hold the srcu read lock in vmx_vcpu_reset path
>>>
>>>   arch/x86/kvm/vmx.c |    2 +-
>>>   1 files changed, 1 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>> index a0f78db..3e556c6 100644
>>> --- a/arch/x86/kvm/vmx.c
>>> +++ b/arch/x86/kvm/vmx.c
>>> @@ -4579,7 +4579,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu)
>>>   		vmcs_write32(TPR_THRESHOLD, 0);
>>>   	}
>>> -	kvm_vcpu_reload_apic_access_page(vcpu);
>>> +	kvm_make_request(KVM_REQ_APIC_PAGE_RELOAD, vcpu);
>>>   	if (vmx_vm_has_apicv(vcpu->kvm))
>>>   		memset(&vmx->pi_desc, 0, sizeof(struct pi_desc));
> .
>