linux-kernel - Re: [RFC PATCH 8/8] locking/lockdep: Use shazptr to protect the key hashlist

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Message-ID: <06bf79cc-bd0d-487c-bcde-44464d17225d@paulmck-laptop>
Date: Thu, 10 Jul 2025 18:04:01 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Breno Leitao <leitao@...ian.org>, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	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>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Uladzislau Rezki <urezki@...il.com>, rcu@...r.kernel.org
Subject: Re: [RFC PATCH 8/8] locking/lockdep: Use shazptr to protect the key
 hashlist

On Sun, Apr 13, 2025 at 11:00:55PM -0700, Boqun Feng wrote:
> Erik Lundgren and Breno Leitao reported [1] a case where
> lockdep_unregister_key() can be called from time critical code pathes
> where rntl_lock() may be held. And the synchronize_rcu() in it can slow
> down operations such as using tc to replace a qdisc in a network device.
> 
> In fact the synchronize_rcu() in lockdep_unregister_key() is to wait for
> all is_dynamic_key() callers to finish so that removing a key from the
> key hashlist, and we can use shazptr to protect the hashlist as well.
> 
> Compared to the proposed solution which replaces synchronize_rcu() with
> synchronize_rcu_expedited(), using shazptr here can achieve the
> same/better synchronization time without the need to send IPI. Hence use
> shazptr here.
> 
> Reported-by: Erik Lundgren <elundgren@...a.com>
> Reported-by: Breno Leitao <leitao@...ian.org>
> Link: https://lore.kernel.org/lkml/20250321-lockdep-v1-1-78b732d195fb@debian.org/
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>

>From an RCU and shazptr viewpoint:

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  kernel/locking/lockdep.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 58d78a33ac65..c5781d2dc8c6 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -58,6 +58,7 @@
>  #include <linux/context_tracking.h>
>  #include <linux/console.h>
>  #include <linux/kasan.h>
> +#include <linux/shazptr.h>
>  
>  #include <asm/sections.h>
>  
> @@ -1265,14 +1266,18 @@ static bool is_dynamic_key(const struct lock_class_key *key)
>  
>  	hash_head = keyhashentry(key);
>  
> -	rcu_read_lock();
> +	/* Need preemption disable for using shazptr. */
> +	guard(preempt)();
> +
> +	/* Protect the list search with shazptr. */
> +	guard(shazptr)(hash_head);
> +
>  	hlist_for_each_entry_rcu(k, hash_head, hash_entry) {
>  		if (k == key) {
>  			found = true;
>  			break;
>  		}
>  	}
> -	rcu_read_unlock();
>  
>  	return found;
>  }
> @@ -6614,7 +6619,7 @@ void lockdep_unregister_key(struct lock_class_key *key)
>  		call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
>  
>  	/* Wait until is_dynamic_key() has finished accessing k->hash_entry. */
> -	synchronize_rcu();
> +	synchronize_shazptr(keyhashentry(key));
>  }
>  EXPORT_SYMBOL_GPL(lockdep_unregister_key);
>  
> -- 
> 2.47.1
>