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Date: Mon, 20 Jul 2020 10:50:00 +0300 From: Ido Schimmel <idosch@...sch.org> To: Jiri Pirko <jiri@...nulli.us> Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org, jhs@...atatu.com, xiyou.wangcong@...il.com, idosch@...lanox.com, mlxsw@...lanox.com Subject: Re: [patch net-next] sched: sch_api: add missing rcu read lock to silence the warning On Mon, Jul 20, 2020 at 09:22:48AM +0200, Jiri Pirko wrote: > From: Jiri Pirko <jiri@...lanox.com> > > 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 <idosch@...lanox.com> > Signed-off-by: Jiri Pirko <jiri@...lanox.com> > --- > 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 >
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