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Date:   Tue, 08 Sep 2020 20:12:40 -0700 (PDT)
Message-Id: <20200908.201240.821919587285621499.davem@davemloft.net>
To:     kuba@kernel.org
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Subject: Re: [PATCH net-next] rcu: prevent RCU_LOCKDEP_WARN() from
 swallowing the condition
From:   David Miller <davem@davemloft.net>
In-Reply-To: <20200908173624.160024-1-kuba@kernel.org>
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        <20200908173624.160024-1-kuba@kernel.org>
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From: Jakub Kicinski <kuba@kernel.org>
Date: Tue,  8 Sep 2020 10:36:24 -0700

> We run into a unused variable warning in bridge code when
> variable is only used inside the condition of
> rcu_dereference_protected().
> 
>  #define mlock_dereference(X, br) \
> 	rcu_dereference_protected(X, lockdep_is_held(&br->multicast_lock))
> 
> Since on builds with CONFIG_PROVE_RCU=n rcu_dereference_protected()
> compiles to nothing the compiler doesn't see the variable use.
> 
> Prevent the warning by adding the condition as dead code.
> We need to un-hide the declaration of lockdep_tasklist_lock_is_held()
> and fix a bug the crept into a net/sched header.
> 
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>

I ended up applying Nikolay's fix, but this situation with the rcu macros
needs to be addressed.