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.mayd...@linaro.org>
Reported-by: Stuart Monteith <stuart.monte...@linaro.org>
---
Based-on: <1512397331-15238-1-git-send-email-peter.mayd...@linaro.org>
(this applies on top of 'linux-user: Fix locking order in fork_start()')
I think this should fix the deadlock that Stuart reports, but I
can't reproduce it, so testing welcome.
linux-user/main.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/linux-user/main.c b/linux-user/main.c
index 146ee3e..ff116fe 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -128,6 +128,7 @@ 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();
@@ -148,9 +149,13 @@ 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();
}
}
--
2.7.4
