linux-kernel - Re: [PATCH] lockdep: Speed up lockdep_unregister

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250324121202.GG14944@noisy.programming.kicks-ass.net>
Date: Mon, 24 Mar 2025 13:12:02 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Breno Leitao <leitao@...ian.org>
Cc: Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>, Waiman Long <longman@...hat.com>,
	aeh@...a.com, linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	edumazet@...gle.com, jhs@...atatu.com, kernel-team@...a.com,
	Erik Lundgren <elundgren@...a.com>,
	"Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] lockdep: Speed up lockdep_unregister_key() with
 expedited RCU synchronization

On Fri, Mar 21, 2025 at 02:30:49AM -0700, Breno Leitao wrote:
> 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
> 
> Reported-by: Erik Lundgren <elundgren@...a.com>
> Signed-off-by: Breno Leitao <leitao@...ian.org>
> Reviewed-by: "Paul E. McKenney" <paulmck@...nel.org>
> ---
>  kernel/locking/lockdep.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 4470680f02269..a79030ac36dd4 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -6595,8 +6595,10 @@ void lockdep_unregister_key(struct lock_class_key *key)
>  	if (need_callback)
>  		call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
>  
> -	/* Wait until is_dynamic_key() has finished accessing k->hash_entry. */
> -	synchronize_rcu();
> +	/* Wait until is_dynamic_key() has finished accessing k->hash_entry.
> +	 * This needs to be quick, since it is called in critical sections
> +	 */
> +	synchronize_rcu_expedited();
>  }
>  EXPORT_SYMBOL_GPL(lockdep_unregister_key);

So I fundamentally despise synchronize_rcu_expedited(), also your
comment style is broken.

Why can't qdisc call this outside of the lock?