From 73c6e4013b4cd92d3d531bc22cc29e6036ef42e0 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Wed, 27 Jan 2016 08:49:21 +0100 Subject: rcu: completely disable pthread_atfork callbacks as soon as possible Because of -daemonize, system mode QEMU sometimes needs to fork() and keep RCU enabled in the child. However, there is a possible deadlock with synchronize_rcu: - the CPU thread is inside a RCU critical section and wants to take the BQL in order to do MMIO - the monitor thread, which is owning the BQL, calls rcu_init_lock which tries to take the rcu_sync_lock - the call_rcu thread has taken rcu_sync_lock in synchronize_rcu, but synchronize_rcu needs the CPU thread to end the critical section before returning. This cannot happen for user-mode emulation, because it does not have a BQL. To fix it, assume that system mode QEMU only forks in preparation for exec (except when daemonizing) and disable pthread_atfork as soon as the double fork has happened. Reported-by: Dr. David Alan Gilbert Tested-by: Dr. David Alan Gilbert Signed-off-by: Paolo Bonzini --- include/qemu/rcu.h | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'include') diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h index 83ae2808be..c0da9907e8 100644 --- a/include/qemu/rcu.h +++ b/include/qemu/rcu.h @@ -105,6 +105,12 @@ extern void synchronize_rcu(void); */ extern void rcu_register_thread(void); extern void rcu_unregister_thread(void); + +/* + * Support for fork(). fork() support is enabled at startup. + */ +extern void rcu_enable_atfork(void); +extern void rcu_disable_atfork(void); extern void rcu_after_fork(void); struct rcu_head; -- cgit v1.2.3-55-g7522