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>
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