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7a3cedafcc
lockdep_unregister_key() is called from critical code paths, including
sections where rtnl_lock() is held. For example, when replacing a qdisc
in a network device, network egress traffic is disabled while
__qdisc_destroy() is called for every network queue.
If lockdep is enabled, __qdisc_destroy() calls lockdep_unregister_key(),
which gets blocked waiting for synchronize_rcu() to complete.
For example, a simple tc command to replace a qdisc could take 13
seconds:
# time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1: mq
real 0m13.195s
user 0m0.001s
sys 0m2.746s
During this time, network egress is completely frozen while waiting for
RCU synchronization.
Use synchronize_rcu_expedited() instead to minimize the impact on
critical operations like network connectivity changes.
This improves 10x the function call to tc, when replacing the qdisc for
a network card.
# time /usr/sbin/tc qdisc replace dev eth0 root handle 0x1: mq
real 0m1.789s
user 0m0.000s
sys 0m1.613s
[boqun: Fixed the comment and add more information for the temporary
workaround, and add TODO information for hazptr]
Reported-by: Erik Lundgren <elundgren@meta.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20250321-lockdep-v1-1-78b732d195fb@debian.org
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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 reStructuredText 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.