linux-kernel - Re: [PATCH] rcu: update: Check rcu_bh_lock_map state in rcu_read_lock_bh

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Date:   Tue, 22 Jun 2021 10:58:55 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Neeraj Upadhyay <neeraju@...eaurora.org>
Cc:     josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, urezki@...il.com, frederic@...nel.org
Subject: Re: [PATCH] rcu: update: Check rcu_bh_lock_map state in
 rcu_read_lock_bh_held

On Tue, Jun 22, 2021 at 05:35:21PM +0530, Neeraj Upadhyay wrote:
> In addition to irq and softirq state, check rcu_bh_lock_map
> state, to decide whether RCU bh lock is held.
> 
> Signed-off-by: Neeraj Upadhyay <neeraju@...eaurora.org>

My initial reaction was that "in_softirq() || irqs_disabled()" covers
it because rcu_read_lock_bh() disables BH.  But you are right that it
does seem a bit silly to ignore lockdep.

So would it also make sense to have a WARN_ON_ONCE() if lockdep claims
we are under rcu_read_lock_bh() protection, but "in_softirq() ||
irqs_disabled()" think otherwise?

							Thanx, Paul

> ---
>  kernel/rcu/update.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index c21b38c..d416f1c 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -333,7 +333,7 @@ int rcu_read_lock_bh_held(void)
>  
>  	if (rcu_read_lock_held_common(&ret))
>  		return ret;
> -	return in_softirq() || irqs_disabled();
> +	return lock_is_held(&rcu_bh_lock_map) || in_softirq() || irqs_disabled();
>  }
>  EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
>  
> -- 
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, 
> hosted by The Linux Foundation
>