Commit 73c6e401 authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

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: default avatarDr. David Alan Gilbert <dgilbert@redhat.com>
Tested-by: default avatarDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent c233a35d

include/qemu/rcu.h

+6 −0
Original line number Diff line number Diff line
@@ -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;

util/rcu.c

+20 −0
Original line number Diff line number Diff line
@@ -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);

}

vl.c

+1 −0
Original line number Diff line number Diff line
@@ -4121,6 +4121,7 @@ int main(int argc, char **argv, char **envp) 
    set_memory_options(&ram_slots, &maxram_size, machine_class);



    os_daemonize();

    rcu_disable_atfork();



    if (pid_file && qemu_create_pidfile(pid_file) != 0) {

        error_report("could not acquire pid file: %s", strerror(errno));