linux-kernel - Re: [PATCH] cfg80211: Pass lockdep expression to RCU lists

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Date:   Wed, 19 Feb 2020 22:29:43 +0100
From:   Johannes Berg <johannes@...solutions.net>
To:     Arend Van Spriel <arend.vanspriel@...adcom.com>,
        Amol Grover <frextrite@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] cfg80211: Pass lockdep expression to RCU lists

On Wed, 2020-02-19 at 22:27 +0100, Arend Van Spriel wrote:
> On 2/19/2020 10:13 AM, Johannes Berg wrote:
> > On Wed, 2020-02-19 at 14:41 +0530, Amol Grover wrote:
> > >   
> > > -	WARN_ON_ONCE(!rcu_read_lock_held() && !lockdep_rtnl_is_held());
> > > -
> > > -	list_for_each_entry_rcu(pos, &rdev->sched_scan_req_list, list) {
> > > +	list_for_each_entry_rcu(pos, &rdev->sched_scan_req_list, list,
> > > +				lockdep_rtnl_is_held()) {
> > 
> > Huh, I didn't even know you _could_ do that :)
> 
> Me neither ;-). Above you are removing the WARN_ON_ONCE() entirely. 
> Would it not be good to keep the WARN_ON_ONCE() with only the 
> !rcu_read_lock_held() check.

Not needed, the macro expansion will already contain
rcu_read_lock_any_held() just like in all the other cases where you pass
a lockdep condition to RCU helpers.

johannes