Re: request_module while holding rtnl semaphore

["David S. Miller" <davem@davemloft.net>]

On Sat, 06 Nov 2004 10:35:45 +1100
Herbert Xu <herbert@gondor.apana.org.au> wrote:

> 1) Abuse of the rtnl.  It's being used for too many things.  It's
> basically the networking system's BKL.  If the locking were more
> granular then this shouldn't occur.

I totally disagree.  The use of rtnl for all networking configuration
changes is a virtue of our current setup.

All of these actions want to make sure devices don't disappear
from underneath them while verifying a configuration change.  In
fact one of the first things the packet scheduler config change
code does is:

	if ((dev = __dev_get_by_index(tcm->tcm_ifindex)) == NULL)
		return -ENODEV;

and it expects the state of that device to not change throughout
the rest of the config change verification and decision making.

> 2) Hooking random net/sched requests into rtnetlink.  By being an
> rtnetlink user you pay the price of taking the rtnl.  Most of the
> net/sched stuff has nothing to do with rtnetlink.

I believe this is a false statement.  Every single networking
config change depends upon device existence and state in some
way.  By virtue of that, they really depend upon the RTNL being
held during the duration of their execution.

Only the actual implementations know when and where such a
dependency on device state et al. does not exist, and therefore
where RTNL holding is not necessary.

Therefore I suggest we just implement the fix for this inside of
the packet scheduler layer itself.  Simply by dropping the RTNL
semaphore during the module request, and then regrabbing the RTNL
semaphore and replaying the request from the beginning.

The net/sched/sch_api.c version of the fix would look like the
following.  The act_api.c case would require a bit more surgery,
but with the right restructuring it can be done too.

===== net/sched/sch_api.c 1.41 vs edited =====
--- 1.41/net/sched/sch_api.c	2004-11-05 16:34:45 -08:00
+++ edited/net/sched/sch_api.c	2004-11-09 15:57:19 -08:00
@@ -396,17 +396,30 @@
 	struct Qdisc_ops *ops;
 	int size;
 
+	err = -EINVAL;
 	ops = qdisc_lookup_ops(kind);
 #ifdef CONFIG_KMOD
 	if (ops==NULL && tca[TCA_KIND-1] != NULL) {
 		if (RTA_PAYLOAD(kind) <= IFNAMSIZ) {
+			rtnl_unlock();
 			request_module("sch_%s", (char*)RTA_DATA(kind));
+			rtnl_lock();
+
 			ops = qdisc_lookup_ops(kind);
+
+			/* We dropped the RTNL semaphore in order to
+			 * perform the module load.  So, even if we
+			 * succeeded in loading the module we have to
+			 * tell the caller to replay the request.  We
+			 * indicate this using -EAGAIN.
+			 */
+			if (ops != NULL)
+				err = -EAGAIN;
+			goto err_out;
 		}
 	}
 #endif
 
-	err = -EINVAL;
 	if (ops == NULL)
 		goto err_out;
 	err = -EBUSY;
@@ -600,14 +613,19 @@
 
 static int tc_modify_qdisc(struct sk_buff *skb, struct nlmsghdr *n, void *arg)
 {
-	struct tcmsg *tcm = NLMSG_DATA(n);
-	struct rtattr **tca = arg;
+	struct tcmsg *tcm;
+	struct rtattr **tca;
 	struct net_device *dev;
-	u32 clid = tcm->tcm_parent;
-	struct Qdisc *q = NULL;
-	struct Qdisc *p = NULL;
+	u32 clid;
+	struct Qdisc *q, *p;
 	int err;
 
+replay:
+	tcm = NLMSG_DATA(n);
+	tca = arg;
+	clid = tcm->tcm_parent;
+	q = p = NULL;
+
 	if ((dev = __dev_get_by_index(tcm->tcm_ifindex)) == NULL)
 		return -ENODEV;
 
@@ -701,8 +719,14 @@
 		q = qdisc_create(dev, tcm->tcm_parent, tca, &err);
         else
 		q = qdisc_create(dev, tcm->tcm_handle, tca, &err);
-	if (q == NULL)
+	if (q == NULL) {
+		if (err == -EAGAIN) {
+			/* Replay the request. */
+			dev_put(dev);
+			goto replay;
+		}
 		return err;
+	}
 
 graft:
 	if (1) {