net/sched: taprio: keep child Qdisc refcount elevated at 2 in offload mode

net/sched: taprio: keep child Qdisc refcount elevated at 2 in offload mode

Normally, Qdiscs have one reference on them held by their owner and one
held for each TXQ to which they are attached, however this is not the
case with the children of an offloaded taprio. Instead, the taprio qdisc
currently lives in the following fragile equilibrium.

In the software scheduling case, taprio attaches itself (the root Qdisc)
to all TXQs, thus having a refcount of 1 + the number of TX queues. In
this mode, the q->qdiscs[] children are not visible directly to the
Qdisc API. The lifetime of the Qdiscs from this private array lasts
until qdisc_destroy() -> taprio_destroy().

In the fully offloaded case, the root taprio has a refcount of 1, and
all child q->qdiscs[] also have a refcount of 1. The child q->qdiscs[]
are attached to the netdev TXQs directly and thus are visible to the
Qdisc API, however taprio loses a reference to them very early - during
qdisc_graft(parent==NULL) -> taprio_attach(). At that time, taprio frees
the q->qdiscs[] array to not leak memory, but interestingly, it does not
release a reference on these qdiscs because it doesn't effectively own
them - they are created by taprio but owned by the Qdisc core, and will
be freed by qdisc_graft(parent==NULL, new==NULL) -> qdisc_put(old) when
the Qdisc is deleted or when the child Qdisc is replaced with something
else.

My interest is to change this equilibrium such that taprio also owns a
reference on the q->qdiscs[] child Qdiscs for the lifetime of the root
Qdisc, including in full offload mode. I want this because I would like
taprio_leaf(), taprio_dump_class(), taprio_dump_class_stats() to have
insight into q->qdiscs[] for the software scheduling mode - currently
they look at dev_queue->qdisc_sleeping, which is, as mentioned, the same
as the root taprio.

The following set of changes is necessary:
- don't free q->qdiscs[] early in taprio_attach(), free it late in
  taprio_destroy() for consistency with software mode. But:
- currently that's not possible, because taprio doesn't own a reference
  on q->qdiscs[]. So hold that reference - once during the initial
  attach() and once during subsequent graft() calls when the child is
  changed.
- always keep track of the current child in q->qdiscs[], even for full
  offload mode, so that we free in taprio_destroy() what we should, and
  not something stale.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20230807193324.4128292-3-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>