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Message-ID: <2d1f4f16-6d66-4449-a7b7-5e2b2a4eaeb3@acm.org>
Date: Thu, 1 Feb 2024 09:22:20 -0800
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Subject: Re: [PATCH V2] lockdep: fix deadlock issue between lockdep and rcu
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To: Zhiguo Niu <zhiguo.niu@unisoc.com>, peterz@infradead.org,
 mingo@redhat.com, will@kernel.org, longman@redhat.com, boqun.feng@gmail.com
Cc: linux-kernel@vger.kernel.org, niuzhiguo84@gmail.com, ke.wang@unisoc.com,
 xuewen.yan@unisoc.com, Carlos Llamas <cmllamas@google.com>
References: <1705477714-10467-1-git-send-email-zhiguo.niu@unisoc.com>
From: Bart Van Assche <bvanassche@acm.org>
In-Reply-To: <1705477714-10467-1-git-send-email-zhiguo.niu@unisoc.com>
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On 1/16/24 23:48, Zhiguo Niu wrote:
> There is a deadlock scenario between lockdep and rcu when
> rcu nocb feature is enabled, just as following call stack:

Is it necessary to support lockdep for this kernel configuration or should we
rather forbid this combination by changing lib/Kconfig.debug?

>   /*
> - * Schedule an RCU callback if no RCU callback is pending. Must be called with
> - * the graph lock held.
> - */
> -static void call_rcu_zapped(struct pending_free *pf)
> +* See if we need to queue an RCU callback, must called with
> +* the lockdep lock held, returns false if either we don't have
> +* any pending free or the callback is already scheduled.
> +* Otherwise, a call_rcu() must follow this function call.
> +*/

Why has the name been changed from "graph lock" into "lockdep lock"? I think
elsewhere in this source file it is called the "graph lock".

>   	/*
> -	 * If there's anything on the open list, close and start a new callback.
> -	 */
> -	call_rcu_zapped(delayed_free.pf + delayed_free.index);
> +	* If there's anything on the open list, close and start a new callback.
> +	*/
> +	if (need_callback)
> +		call_rcu(&delayed_free.rcu_head, free_zapped_rcu);

The comment above the if-statement refers to the call_rcu_zapped() function
while call_rcu_zapped() has been changed into call_rcu(). So the comment is
now incorrect.

Additionally, what guarantees that the above code won't be triggered
concurrently from two different threads? As you may know calling call_rcu()
twice before the callback has been started is not allowed. I think that can
happen with the above code.

Bart.