KVM: x86: fix deadlock in clock-in-progress request handling

There is a deadlock in pvclock handling:

cpu0:                                               cpu1:
kvm_gen_update_masterclock()
                                              kvm_guest_time_update()
 spin_lock(pvclock_gtod_sync_lock)
                                               local_irq_save(flags)

spin_lock(pvclock_gtod_sync_lock)

 kvm_make_mclock_inprogress_request(kvm)
  make_all_cpus_request()
   smp_call_function_many()

Now if smp_call_function_many() called by cpu0 tries to call function on
cpu1 there will be a deadlock.

Fix by moving pvclock_gtod_sync_lock protected section outside irq
disabled section.

Analyzed by Gleb Natapov <gleb@redhat.com>
Acked-by: Gleb Natapov <gleb@redhat.com>
Reported-and-Tested-by: Yongjie Ren <yongjie.ren@intel.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 9 insertions, 9 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f71500af1f81..f7c850b36910 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1416,15 +1416,6 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
         kernel_ns = 0;
         host_tsc = 0;
 
-        /* Keep irq disabled to prevent changes to the clock */
-        local_irq_save(flags);
-        this_tsc_khz = __get_cpu_var(cpu_tsc_khz);
-        if (unlikely(this_tsc_khz == 0)) {
-                local_irq_restore(flags);
-                kvm_make_request(KVM_REQ_CLOCK_UPDATE, v);
-                return 1;
-        }
-
         /*
          * If the host uses TSC clock, then passthrough TSC as stable
          * to the guest.
@@ -1436,6 +1427,15 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
                 kernel_ns = ka->master_kernel_ns;
         }
         spin_unlock(&ka->pvclock_gtod_sync_lock);
+
+        /* Keep irq disabled to prevent changes to the clock */
+        local_irq_save(flags);
+        this_tsc_khz = __get_cpu_var(cpu_tsc_khz);
+        if (unlikely(this_tsc_khz == 0)) {
+                local_irq_restore(flags);
+                kvm_make_request(KVM_REQ_CLOCK_UPDATE, v);
+                return 1;
+        }
         if (!use_master_clock) {
                 host_tsc = native_read_tsc();
                 kernel_ns = get_kernel_ns();