We were calling rcu_init_complete() twice in the child processes when fork happened. However the pthread library does not really suggest to do it that way:
http://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_mutex_init.html "Attempting to initialise an already initialised mutex results in undefined behaviour." Actually, IMHO we can do it in a more natural way: Firstly, we only init the RCU globals once in rcu_init(). Then, in rcu_init_child(), we unlock all the locks held in rcu_init_lock() just like what we do in the parent process, then do the rest of RCU re-init (e.g., create the RCU thread). CC: Paolo Bonzini <pbonz...@redhat.com> Signed-off-by: Peter Xu <pet...@redhat.com> --- this is based on Paolo's series: "[PATCH for-2.10 0/2] RCU: forking fix and cleanups" --- util/rcu.c | 18 ++++++++++++++---- 1 file changed, 14 insertions(+), 4 deletions(-) diff --git a/util/rcu.c b/util/rcu.c index ca5a63e..6fbbe4c 100644 --- a/util/rcu.c +++ b/util/rcu.c @@ -299,15 +299,17 @@ void rcu_unregister_thread(void) qemu_mutex_unlock(&rcu_registry_lock); } -static void rcu_init_complete(void) +static void rcu_init_globals(void) { - QemuThread thread; - qemu_mutex_init(&rcu_registry_lock); qemu_mutex_init(&rcu_sync_lock); qemu_event_init(&rcu_gp_event, true); - qemu_event_init(&rcu_call_ready_event, false); +} + +static void rcu_init_complete(void) +{ + QemuThread thread; /* The caller is assumed to have iothread lock, so the call_rcu thread * must have been quiescent even after forking, just recreate it. @@ -357,6 +359,13 @@ static void rcu_init_child(void) return; } + rcu_init_unlock(); + + /* + * For the newly forked child, we need something extra: since + * after fork the threads are all gone, we need to re-init the RCU + * thread, along with the globals. + */ memset(®istry, 0, sizeof(registry)); rcu_init_complete(); } @@ -367,5 +376,6 @@ static void __attribute__((__constructor__)) rcu_init(void) #ifdef CONFIG_POSIX pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_child); #endif + rcu_init_globals(); rcu_init_complete(); } -- 2.7.4
