linux-user: wrap fork() in a start/end exclusive section

When we do a fork() in usermode emulation, we need to be in
a start/end exclusive section, so that we can ensure that no
other thread is in an RCU section. Otherwise you can get this
deadlock:

- fork thread: has mmap_lock, waits for rcu_sync_lock
  (because rcu_init_lock() is registered as a pthread_atfork() hook)
- RCU thread: has rcu_sync_lock, waits for rcu_read_(un)lock
- another CPU thread: in RCU critical section, waits for mmap_lock

This can show up if you have a heavily multithreaded guest program
that does a fork().
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reported-by: NStuart Monteith <stuart.monteith@linaro.org>
Message-Id: <1512650481-1723-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

linux-user/main.c

@@ -127,6 +127,7 @@ int cpu_get_pic_interrupt(CPUX86State *env)
 /* Make sure everything is in a consistent state for calling fork().  */
 void fork_start(void)
 {
+    start_exclusive();
     mmap_fork_start();
     qemu_mutex_lock(&tb_ctx.tb_lock);
     cpu_list_lock();
@@ -147,9 +148,13 @@ void fork_end(int child)
         qemu_mutex_init(&tb_ctx.tb_lock);
         qemu_init_cpu_list();
         gdbserver_fork(thread_cpu);
+        /* qemu_init_cpu_list() takes care of reinitializing the
+         * exclusive state, so we don't need to end_exclusive() here.
+         */
     } else {
         qemu_mutex_unlock(&tb_ctx.tb_lock);
         cpu_list_unlock();
+        end_exclusive();
     }
 }