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DSA is preparing to offer switch drivers an API through which they can
associate each FDB entry with a struct net_device *bridge_dev. This can
be used to perform FDB isolation (the FDB lookup performed on the
ingress of a standalone, or bridged port, should not find an FDB entry
that is present in the FDB of another bridge).
In preparation of that work, DSA needs to ensure that by the time we
call the switch .port_fdb_add and .port_fdb_del methods, the
dp->bridge_dev pointer is still valid, i.e. the port is still a bridge
port.
This is not guaranteed because the SWITCHDEV_FDB_{ADD,DEL}_TO_DEVICE API
requires drivers that must have sleepable context to handle those events
to schedule the deferred work themselves. DSA does this through the
dsa_owq.
It can happen that a port leaves a bridge, del_nbp() flushes the FDB on
that port, SWITCHDEV_FDB_DEL_TO_DEVICE is notified in atomic context,
DSA schedules its deferred work, but del_nbp() finishes unlinking the
bridge as a master from the port before DSA's deferred work is run.
Fundamentally, the port must not be unlinked from the bridge until all
FDB deletion deferred work items have been flushed. The bridge must wait
for the completion of these hardware accesses.
An attempt has been made to address this issue centrally in switchdev by
making SWITCHDEV_FDB_DEL_TO_DEVICE deferred (=> blocking) at the switchdev
level, which would offer implicit synchronization with del_nbp:
https://patchwork.kernel.org/project/netdevbpf/cover/20210820115746.3701811-1-vladimir.oltean@nxp.com/
but it seems that any attempt to modify switchdev's behavior and make
the events blocking there would introduce undesirable side effects in
other switchdev consumers.
The most undesirable behavior seems to be that
switchdev_deferred_process_work() takes the rtnl_mutex itself, which
would be worse off than having the rtnl_mutex taken individually from
drivers which is what we have now (except DSA which has removed that
lock since commit
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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.