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: Peter Maydell <peter.maydell@linaro.org>
Reported-by: Stuart Monteith <stuart.monteith@linaro.org>
Message-Id: <1512650481-1723-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
diff --git a/linux-user/main.c b/linux-user/main.c
index e840691..2140465 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -127,6 +127,7 @@
/* 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 @@
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();
}
}
