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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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|---|---|---|
| .. | ||
| irqflag-debug.c | ||
| lock_events.c | ||
| lock_events.h | ||
| lock_events_list.h | ||
| lockdep.c | ||
| lockdep_internals.h | ||
| lockdep_proc.c | ||
| lockdep_states.h | ||
| locktorture.c | ||
| Makefile | ||
| mcs_spinlock.h | ||
| mutex-debug.c | ||
| mutex.c | ||
| mutex.h | ||
| osq_lock.c | ||
| percpu-rwsem.c | ||
| qrwlock.c | ||
| qspinlock.c | ||
| qspinlock.h | ||
| qspinlock_paravirt.h | ||
| qspinlock_stat.h | ||
| rtmutex.c | ||
| rtmutex_api.c | ||
| rtmutex_common.h | ||
| rwbase_rt.c | ||
| rwsem.c | ||
| semaphore.c | ||
| spinlock.c | ||
| spinlock_debug.c | ||
| spinlock_rt.c | ||
| test-ww_mutex.c | ||
| ww_mutex.h | ||
| ww_rt_mutex.c | ||