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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>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.