Subject: Re: [patch net-next] sched: sch_api: add missing rcu read lock to silence the warning

From: Ido Schimmel <[email-masked]>
Date: Mon, 20 Jul 2020 10:50:00 +0300

On Mon, Jul 20, 2020 at 09:22:48AM +0200, Jiri Pirko wrote:
> From: Jiri Pirko <[email-masked]>
> 
> In case the qdisc_match_from_root function() is called from non-rcu path
> with rtnl mutex held, a suspiciout rcu usage warning appears:
> 
> [  241.504354] =============================
> [  241.504358] WARNING: suspicious RCU usage
> [  241.504366] 5.8.0-rc4-custom-01521-g72a7c7d549c3 #32 Not tainted
> [  241.504370] -----------------------------
> [  241.504378] net/sched/sch_api.c:270 RCU-list traversed in non-reader section!!
> [  241.504382]
>                other info that might help us debug this:
> [  241.504388]
>                rcu_scheduler_active = 2, debug_locks = 1
> [  241.504394] 1 lock held by tc/1391:
> [  241.504398]  #0: ffffffff85a27850 (rtnl_mutex){+.+.}-{3:3}, at: rtnetlink_rcv_msg+0x49a/0xbd0
> [  241.504431]
>                stack backtrace:
> [  241.504440] CPU: 0 PID: 1391 Comm: tc Not tainted 5.8.0-rc4-custom-01521-g72a7c7d549c3 #32
> [  241.504446] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-2.fc32 04/01/2014
> [  241.504453] Call Trace:
> [  241.504465]  dump_stack+0x100/0x184
> [  241.504482]  lockdep_rcu_suspicious+0x153/0x15d
> [  241.504499]  qdisc_match_from_root+0x293/0x350
> 
> Fix this by taking the rcu_lock for qdisc_hash iteration.
> 
> Reported-by: Ido Schimmel <[email-masked]>
> Signed-off-by: Jiri Pirko <[email-masked]>
> ---
>  net/sched/sch_api.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
> index 11ebba60da3b..c7cfd8dc6a77 100644
> --- a/net/sched/sch_api.c
> +++ b/net/sched/sch_api.c
> @@ -267,10 +267,12 @@ static struct Qdisc *qdisc_match_from_root(struct Qdisc *root, u32 handle)
>  	    root->handle == handle)
>  		return root;
>  
> +	rcu_read_lock();
>  	hash_for_each_possible_rcu(qdisc_dev(root)->qdisc_hash, q, hash, handle) {
>  		if (q->handle == handle)
>  			return q;

You don't unlock here, but I'm not sure it's the best fix. It's weird to
return an object from an RCU critical section without taking a
reference. It can also hide a bug if someone calls
qdisc_match_from_root() without RTNL or RCU.

hash_for_each_possible_rcu() is basically hlist_for_each_entry_rcu()
which already accepts:

@cond:       optional lockdep expression if called from non-RCU protection.

So maybe extend hash_for_each_possible_rcu() with 'cond' and pass a
lockdep expression to see if RTNL is held?

>  	}
> +	rcu_read_unlock();
>  	return NULL;
>  }
> -- 
> 2.21.3