Messages in this thread | | | Date | Mon, 08 Jun 2009 19:00:10 +0200 | From | Oliver Hartkopp <> | Subject | Re: [PATCH 3/5] can: af_can.c use rcu_barrier() on module unload. |
| |
Paul E. McKenney wrote: > On Mon, Jun 08, 2009 at 03:11:38PM +0200, Jesper Dangaard Brouer wrote: >> This module uses rcu_call() thus it should use rcu_barrier() >> on module unload. > > This does appear to make things better!!! > > However, I don't understand why it is safe to do the following in > can_exit(): > > hlist_for_each_entry_safe(d, n, next, &can_rx_dev_list, list) { > hlist_del(&d->list); > kfree(d); > } > > Given that this list is scanned by RCU readers, shouldn't this kfree() > be something like "call_rcu(&d->rcu, can_rx_delete_device);"? > > Also, what frees up the "struct receiver" structures?
Hi Paul,
af_can.c only provides an infrastructure for PF_CAN modules like can-raw.ko, can-bcm.ko or can-isotp.ko.
Please take a look into can_notifier() in net/can/af_can.c and raw_notifier() in net/can/raw.c:
The receivers are removed when the appropriate socket is closed that created the belonging receivers. And you can not remove can.ko (af_can.c) when another PF_CAN protocol like can-raw.ko is using it.
So when a netdev notifier removes the interface both the PF_CAN protocol (e.g. can-raw.ko) and the PF_CAN core (af_can.c) cleans up all receivers and finally removes the per-interface structure dev_rcv_lists (e.g. for can0).
In can_exit() all the dev_rcv_lists for ARPHRD_CAN interfaces are removed that had been created by NETDEV_REGISTER notifier and are unused by any of the PF_CAN protocols and therefore without any receivers attached to them.
The list is protected by spin_lock(&can_rcvlists_lock) - which is probably not even needed in this particular case - and there is no PF_CAN protocol registered at this time. So it's really save to remove the empty dev_rcv_lists structs here that do not link to any receivers.
Puh - much text. But i hope it clarifies it.
Thinking about the rcu stuff again, rcu_barrier() still makes sense when you are unloading the module chain of can-raw.ko and can.ko very fast.
Regards, Oliver
>> Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk> >> --- >> >> net/can/af_can.c | 2 ++ >> 1 files changed, 2 insertions(+), 0 deletions(-) >> >> diff --git a/net/can/af_can.c b/net/can/af_can.c >> index 10f0528..e733725 100644 >> --- a/net/can/af_can.c >> +++ b/net/can/af_can.c >> @@ -903,6 +903,8 @@ static __exit void can_exit(void) >> } >> spin_unlock(&can_rcvlists_lock); >> >> + rcu_barrier(); /* Wait for completion of call_rcu()'s */ >> + >> kmem_cache_destroy(rcv_cache); >> }
| |