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rcu: completely disable pthread_atfork callbacks as soon as possible

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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>
This commit is contained in:
Paolo Bonzini 2016-01-27 08:49:21 +01:00
parent c233a35d3d
commit 73c6e4013b
3 changed files with 27 additions and 0 deletions
Download patch file Download diff file Expand all files Collapse all files

20
util/rcu.c
View file

@ -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);
}