On Thu, 17 May 2012 07:06:52 +0400 Glauber Costa <glommer@xxxxxxxxxxxxx> wrote:
> ...
> >> + else if (val != RESOURCE_MAX) {
> >> + /*
> >> + * ->activated needs to be written after the static_key update.
> >> + * This is what guarantees that the socket activation function
> >> + * is the last one to run. See sock_update_memcg() for details,
> >> + * and note that we don't mark any socket as belonging to this
> >> + * memcg until that flag is up.
> >> + *
> >> + * We need to do this, because static_keys will span multiple
> >> + * sites, but we can't control their order. If we mark a socket
> >> + * as accounted, but the accounting functions are not patched in
> >> + * yet, we'll lose accounting.
> >> + *
> >> + * We never race with the readers in sock_update_memcg(), because
> >> + * when this value change, the code to process it is not patched in
> >> + * yet.
> >> + */
> >> + if (!cg_proto->activated) {
> >> + static_key_slow_inc(&memcg_socket_limit_enabled);
> >> + cg_proto->activated = true;
> >> + }
> >
> > If two threads run this code concurrently, they can both see
> > cg_proto->activated==false and they will both run
> > static_key_slow_inc().
> >
> > Hopefully there's some locking somewhere which prevents this, but it is
> > unobvious. We should comment this, probably at the cg_proto.activated
> > definition site. Or we should fix the bug ;)
> >
> If that happens, locking in static_key_slow_inc will prevent any damage.
> My previous version had explicit code to prevent that, but we were
> pointed out that this is already part of the static_key expectations, so
> that was dropped.
This makes no sense. If two threads run that code concurrently,
key->enabled gets incremented twice. Nobody anywhere has a record that
this happened so it cannot be undone. key->enabled is now in an
unknown state.
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