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 <dgilbert@redhat.com>
Tested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/util/rcu.c b/util/rcu.c
index 9adc5e4a36..2142ddd93b 100644
--- a/util/rcu.c
+++ b/util/rcu.c
@@ -318,15 +318,35 @@ static void rcu_init_complete(void)
     rcu_register_thread();
 }
 
+static int atfork_depth = 1;
+
+void rcu_enable_atfork(void)
+{
+    atfork_depth++;
+}
+
+void rcu_disable_atfork(void)
+{
+    atfork_depth--;
+}
+
 #ifdef CONFIG_POSIX
 static void rcu_init_lock(void)
 {
+    if (atfork_depth < 1) {
+        return;
+    }
+
     qemu_mutex_lock(&rcu_sync_lock);
     qemu_mutex_lock(&rcu_registry_lock);
 }
 
 static void rcu_init_unlock(void)
 {
+    if (atfork_depth < 1) {
+        return;
+    }
+
     qemu_mutex_unlock(&rcu_registry_lock);
     qemu_mutex_unlock(&rcu_sync_lock);
 }