On 11/05/2017 14:00, 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 > RSP: 002b:00007f9d0f6768f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010 > RAX: ffffffffffffffda RBX: ffffffffa64d53c3 RCX: 00007f9d164ed357 > RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 000000000000000d > RBP: ffffbb260856bf88 R08: 0000556b2a13eeb0 R09: 0000000000000000 > R10: 00007f9d080000c8 R11: 0000000000000246 R12: 0000000000000000 > R13: 00007f9d1853d000 R14: 0000000000000000 R15: 000000000000ae80 > ? __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 fix it by holding pvclock_gtod_sync_lock lock when calculates > pvclock's time scale in order to disable preemption for host per-CPU tsc > rate read. > > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> > Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx> > Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> > --- > arch/x86/kvm/x86.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index b54125b..8008d56 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -1772,11 +1772,11 @@ u64 get_kvmclock_ns(struct kvm *kvm) > > hv_clock.tsc_timestamp = ka->master_cycle_now; > hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset; > - spin_unlock(&ka->pvclock_gtod_sync_lock); > > kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL, > &hv_clock.tsc_shift, > &hv_clock.tsc_to_system_mul); > + spin_unlock(&ka->pvclock_gtod_sync_lock); > return __pvclock_read_cycles(&hv_clock, rdtsc()); > } > > This would not be enough for PREEMPT_RT. You need to use get_cpu/put_cpu (including __pvclock_read_cycles in the non-preemptable section). Thanks, Paolo
