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

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

Hi Wanpeng,

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.
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));