[Qemu-devel] Re: [PATCH] kvm: Prevent dynticks race condition for !CONFIG_IOTHREAD

[Jan Kiszka]

On 2011-01-26 10:39, Stefan Hajnoczi wrote:
> The dynticks timer arranges for SIGALRM to be raised when the next
> pending timer expires.  When building with !CONFIG_IOTHREAD, we need to
> check whether a request to exit the vcpu is pending before re-entering
> the guest.
> 
> Unfortunately there is a race condition here because SIGALRM may be
> raised after we check for an exit request but before re-entering the
> guest.  In that case the guest is re-entered without the dynticks timer
> being rearmed.
> 
> This results in temporary loss of timers until some other event forces a
> vmexit.  In the case of a CPU-bound guest it can cause softlockups.
> 
> This patch blocks SIGALRM before checking for an exit request and uses
> KVM's sigmask support to atomically unblock it when entering the guest,
> thereby making the exit request check safe.
> 
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  cpus.c    |   17 ++++++++++++++++-
>  kvm-all.c |   16 ++++++++++++++++
>  2 files changed, 32 insertions(+), 1 deletions(-)
> 
> Does not affect qemu-kvm.git.  Still worth having in qemu.git so we don't get
> odd behavior when building without --enable-io-thread.
> 
> diff --git a/cpus.c b/cpus.c
> index 0309189..59dbfab 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -262,14 +262,29 @@ void qemu_main_loop_start(void)
>  {
>  }
>  
> +static void kvm_init_sigmask(CPUState *env)
> +{
> +    int r;
> +    sigset_t set;
> +
> +    pthread_sigmask(SIG_SETMASK, NULL, &set);
> +    r = kvm_set_signal_mask(env, &set);
> +    if (r) {
> +        fprintf(stderr, "kvm_set_signal_mask: %s\n", strerror(r));
> +        exit(1);
> +    }
> +}
> +
>  void qemu_init_vcpu(void *_env)
>  {
>      CPUState *env = _env;
>  
>      env->nr_cores = smp_cores;
>      env->nr_threads = smp_threads;
> -    if (kvm_enabled())
> +    if (kvm_enabled()) {
>          kvm_init_vcpu(env);
> +        kvm_init_sigmask(env);
> +    }
>      return;
>  }
>  
> diff --git a/kvm-all.c b/kvm-all.c
> index 255b6fa..9cc2553 100644
> --- a/kvm-all.c
> +++ b/kvm-all.c
> @@ -890,19 +890,31 @@ int kvm_cpu_exec(CPUState *env)
>  {
>      struct kvm_run *run = env->kvm_run;
>      int ret;
> +#ifndef CONFIG_IOTHREAD
> +    sigset_t set, old_set;
> +
> +    sigemptyset(&set);
> +    sigaddset(&set, SIGALRM);
> +#endif
>  
>      DPRINTF("kvm_cpu_exec()\n");
>  
>      do {
>  #ifndef CONFIG_IOTHREAD
> +        pthread_sigmask(SIG_BLOCK, &set, &old_set);
> +
>          if (env->exit_request) {
>              DPRINTF("interrupt exit requested\n");
> +            pthread_sigmask(SIG_SETMASK, &old_set, NULL);
>              ret = 0;
>              break;
>          }
>  #endif
>  
>          if (kvm_arch_process_irqchip_events(env)) {
> +#ifndef CONFIG_IOTHREAD
> +            pthread_sigmask(SIG_SETMASK, &old_set, NULL);
> +#endif
>              ret = 0;
>              break;
>          }
> @@ -920,6 +932,10 @@ int kvm_cpu_exec(CPUState *env)
>          cpu_single_env = env;
>          kvm_arch_post_run(env, run);
>  
> +#ifndef CONFIG_IOTHREAD
> +        pthread_sigmask(SIG_SETMASK, &old_set, NULL);
> +#endif
> +
>          if (ret == -EINTR || ret == -EAGAIN) {
>              cpu_exit(env);
>              DPRINTF("io window exit\n");

Good catch, but the code should be organized differently, probably with
the help of signalfd which we need for SIGBUS anyway. I'm currently
reworking signaling bits. I will pick this up and integrate a
corresponding solution in my queue. Will keep you posted!

Jan

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