On 12/05/2017 03:12, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>
> BUG: using __this_cpu_read() in preemptible [00000000] code: qemu-system-x86/2809
> caller is __this_cpu_preempt_check+0x13/0x20
> CPU: 2 PID: 2809 Comm: qemu-system-x86 Not tainted 4.11.0+ #13
> Call Trace:
> dump_stack+0x99/0xce
> check_preemption_disabled+0xf5/0x100
> __this_cpu_preempt_check+0x13/0x20
> get_kvmclock_ns+0x6f/0x110 [kvm]
> get_time_ref_counter+0x5d/0x80 [kvm]
> kvm_hv_process_stimers+0x2a1/0x8a0 [kvm]
> ? kvm_hv_process_stimers+0x2a1/0x8a0 [kvm]
> ? kvm_arch_vcpu_ioctl_run+0xac9/0x1ce0 [kvm]
> kvm_arch_vcpu_ioctl_run+0x5bf/0x1ce0 [kvm]
> kvm_vcpu_ioctl+0x384/0x7b0 [kvm]
> ? kvm_vcpu_ioctl+0x384/0x7b0 [kvm]
> ? __fget+0xf3/0x210
> do_vfs_ioctl+0xa4/0x700
> ? __fget+0x114/0x210
> SyS_ioctl+0x79/0x90
> entry_SYSCALL_64_fastpath+0x23/0xc2
> RIP: 0033:0x7f9d164ed357
> ? __this_cpu_preempt_check+0x13/0x20
>
> This can be reproduced by run kvm-unit-tests/hyperv_stimer.flat w/
> CONFIG_PREEMPT and CONFIG_DEBUG_PREEMPT enabled.
>
> Safe access to per-CPU data requires a couple of constraints, though: the
> thread working with the data cannot be preempted and it cannot be migrated
> while it manipulates per-CPU variables. If the thread is preempted, the
> thread that replaces it could try to work with the same variables; migration
> to another CPU could also cause confusion. However there is no preemption
> disable when reads host per-CPU tsc rate to calculate the current kvmclock
> timestamp.
>
> This patch fixes it by utilizing get_cpu/put_cpu pair to guarantee both
> __this_cpu_read() and rdtsc() are not preempted.
>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
> v2 -> v3:
> * update patch description
> v1 -> v2:
> * utilize get_cpu/put_cpu pair to protect both __this_cpu_read and rdtsc()
>
> arch/x86/kvm/x86.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b54125b..3b5fc7e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1763,6 +1763,7 @@ u64 get_kvmclock_ns(struct kvm *kvm)
> {
> struct kvm_arch *ka = &kvm->arch;
> struct pvclock_vcpu_time_info hv_clock;
> + u64 ret;
>
> spin_lock(&ka->pvclock_gtod_sync_lock);
> if (!ka->use_master_clock) {
> @@ -1774,10 +1775,17 @@ u64 get_kvmclock_ns(struct kvm *kvm)
> hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
> spin_unlock(&ka->pvclock_gtod_sync_lock);
>
> + /* both __this_cpu_read() and rdtsc() should be on the same cpu */
> + get_cpu();
> +
> kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
> &hv_clock.tsc_shift,
> &hv_clock.tsc_to_system_mul);
> - return __pvclock_read_cycles(&hv_clock, rdtsc());
> + ret = __pvclock_read_cycles(&hv_clock, rdtsc());
> +
> + put_cpu();
> +
> + return ret;
> }
>
> static void kvm_setup_pvclock_page(struct kvm_vcpu *v)
>
Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx