cpus: fix deadlock and segfault in qemu_mutex_lock_iothread

When two threads (other than the low-priority TCG VCPU thread)
are competing for the iothread lock, a deadlock can happen.  This
is because iothread_requesting_mutex is set to false by the first
thread that gets the mutex, and then the VCPU thread might never
yield from the execution loop.  If iothread_requesting_mutex is
changed from a bool to a counter, the deadlock is fixed.

However, there is another bug in qemu_mutex_lock_iothread that
can be triggered by the new call_rcu thread.  The bug happens
if qemu_mutex_lock_iothread is called before the CPUs are
created.  In that case, first_cpu is NULL and the caller
segfaults in qemu_mutex_lock_iothread.  To fix this, just
do not do the kick if first_cpu is NULL.
Reported-by: NLeon Alrae <leon.alrae@imgtec.com>
Reported-by: NAndreas Gustafsson <gson@gson.org>
Tested-by: NLeon Alrae <leon.alrae@imgtec.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

cpus.c

@@ -778,7 +778,7 @@ static void qemu_tcg_init_cpu_signals(void)
 
 static QemuMutex qemu_global_mutex;
 static QemuCond qemu_io_proceeded_cond;
-static bool iothread_requesting_mutex;
+static unsigned iothread_requesting_mutex;
 
 static QemuThread io_thread;
 
@@ -1115,15 +1115,15 @@ bool qemu_in_vcpu_thread(void)
 
 void qemu_mutex_lock_iothread(void)
 {
-    if (!tcg_enabled()) {
+    if (!tcg_enabled() || !first_cpu) {
         qemu_mutex_lock(&qemu_global_mutex);
     } else {
-        iothread_requesting_mutex = true;
+        atomic_inc(&iothread_requesting_mutex);
         if (qemu_mutex_trylock(&qemu_global_mutex)) {
             qemu_cpu_kick_thread(first_cpu);
             qemu_mutex_lock(&qemu_global_mutex);
         }
-        iothread_requesting_mutex = false;
+        atomic_dec(&iothread_requesting_mutex);
         qemu_cond_broadcast(&qemu_io_proceeded_cond);
     }
 }