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

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

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 <stefanha@linux.vnet.ibm.com>
---
 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");
-- 
1.7.2.3