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: <20250321-olivine-harrier-of-cubism-f2ad98@leitao>
Date: Fri, 21 Mar 2025 07:22:28 -0700
From: Breno Leitao <leitao@...ian.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, 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,
	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 11:37:31AM +0100, Eric Dumazet wrote:
> On Fri, Mar 21, 2025 at 10:31 AM Breno Leitao <leitao@...ian.org> 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.
> 
> Running 'critical operations' with LOCKDEP enabled kernels seems a bit
> strange :)

Apologies, I meant to say that at Meta, we have certain service tiers
that can accommodate the performance trade-offs of running a "debug"
kernel. This kernel provides valuable feedback about its operations.

The aim is to identify any significant issues early on, rather than
having to debug "hard issues" (such as deadlocks, out-of-memory access,
etc) once the kernel is in production.

> Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Thanks for your support during this investigation,
--breno