From b12a4efb76e81644e290d962d2eb6dbb0a5f214f Mon Sep 17 00:00:00 2001 From: Jose Ricardo Ziviani Date: Sun, 2 Sep 2018 11:19:04 -0300 Subject: [PATCH] Fix a deadlock case in the CPU hotplug flow We need to set cs->halted to 1 before calling ppc_set_compat. The reason is that ppc_set_compat kicks up the new thread created to manage the hotplugged KVM virtual CPU and the code drives directly to KVM_RUN ioctl. When cs->halted is 1, the code: int kvm_cpu_exec(CPUState *cpu) ... if (kvm_arch_process_async_events(cpu)) { atomic_set(&cpu->exit_request, 0); return EXCP_HLT; } ... returns before it reaches KVM_RUN, giving time to the main thread to finish its job. Otherwise we can fall in a deadlock because the KVM thread will issue the KVM_RUN ioctl while the main thread is setting up KVM registers. Depending on how these jobs are scheduled we'll end up freezing QEMU. The following output shows kvm_vcpu_ioctl sleeping because it cannot get the mutex and never will. PS: kvm_vcpu_ioctl was triggered kvm_set_one_reg - compat_pvr. STATE: TASK_UNINTERRUPTIBLE|TASK_WAKEKILL PID: 61564 TASK: c000003e981e0780 CPU: 48 COMMAND: "qemu-system-ppc" #0 [c000003e982679a0] __schedule at c000000000b10a44 #1 [c000003e98267a60] schedule at c000000000b113a8 #2 [c000003e98267a90] schedule_preempt_disabled at c000000000b11910 #3 [c000003e98267ab0] __mutex_lock at c000000000b132ec #4 [c000003e98267bc0] kvm_vcpu_ioctl at c00800000ea03140 [kvm] #5 [c000003e98267d20] do_vfs_ioctl at c000000000407d30 #6 [c000003e98267dc0] ksys_ioctl at c000000000408674 #7 [c000003e98267e10] sys_ioctl at c0000000004086f8 #8 [c000003e98267e30] system_call at c00000000000b488 crash> struct -x kvm.vcpus 0xc000003da0000000 vcpus = {0xc000003db4880000, 0xc000003d52b80000, 0xc0000039e9c80000, 0xc000003d0e200000, 0xc000003d58280000, 0x0, 0x0, ...} crash> struct -x kvm_vcpu.mutex.owner 0xc000003d58280000 mutex.owner = { counter = 0xc000003a23a5c881 <- flag 1: waiters }, crash> bt 0xc000003a23a5c880 PID: 61579 TASK: c000003a23a5c880 CPU: 9 COMMAND: "CPU 4/KVM" (active) crash> struct -x kvm_vcpu.mutex.wait_list 0xc000003d58280000 mutex.wait_list = { next = 0xc000003e98267b10, prev = 0xc000003e98267b10 }, crash> struct -x mutex_waiter.task 0xc000003e98267b10 task = 0xc000003e981e0780 The following command-line was used to reproduce the problem (note: gdb and trace can change the results). $ qemu-ppc/build/ppc64-softmmu/qemu-system-ppc64 -cpu host \ -enable-kvm -m 4096 \ -smp 4,maxcpus=8,sockets=1,cores=2,threads=4 \ -display none -nographic \ -drive file=disk1.qcow2,format=qcow2 ... (qemu) device_add host-spapr-cpu-core,core-id=4 [no interaction is possible after it, only SIGKILL to take the terminal back] Signed-off-by: Jose Ricardo Ziviani Reviewed-by: Greg Kurz Signed-off-by: David Gibson --- hw/ppc/spapr_cpu_core.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/hw/ppc/spapr_cpu_core.c b/hw/ppc/spapr_cpu_core.c index 876f0b3d9d..a73b244a3f 100644 --- a/hw/ppc/spapr_cpu_core.c +++ b/hw/ppc/spapr_cpu_core.c @@ -34,16 +34,16 @@ static void spapr_cpu_reset(void *opaque) cpu_reset(cs); - /* Set compatibility mode to match the boot CPU, which was either set - * by the machine reset code or by CAS. This should never fail. - */ - ppc_set_compat(cpu, POWERPC_CPU(first_cpu)->compat_pvr, &error_abort); - /* All CPUs start halted. CPU0 is unhalted from the machine level * reset code and the rest are explicitly started up by the guest * using an RTAS call */ cs->halted = 1; + /* Set compatibility mode to match the boot CPU, which was either set + * by the machine reset code or by CAS. This should never fail. + */ + ppc_set_compat(cpu, POWERPC_CPU(first_cpu)->compat_pvr, &error_abort); + env->spr[SPR_HIOR] = 0; lpcr = env->spr[SPR_LPCR];