From mboxrd@z Thu Jan  1 00:00:00 1970
From: Cong Wang <cwang@twopensource.com>
Subject: Re: Suspicious RCU usage in bridge with Linux v4.0-9362-g1fc149933fd4
Date: Thu, 23 Apr 2015 15:53:34 -0700
Message-ID: <CAHA+R7P2GcGYMooAhAuo1CeNotc9P4QQTwDyFr-E9O04hcoB0Q@mail.gmail.com>
References: <CA+5PVA5gE_pG5iPKBgugsEjLsOYvq9bWKfB+2OeEujjdOb+URg@mail.gmail.com>
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Cc: Stephen Hemminger <stephen@networkplumber.org>,
	"David S. Miller" <davem@davemloft.net>,
	"bridge@lists.linux-foundation.org"
	<bridge@lists.linux-foundation.org>,
	netdev <netdev@vger.kernel.org>,
	"Linux-Kernel@Vger. Kernel. Org" <linux-kernel@vger.kernel.org>,
	dac.override@gmail.com
To: Josh Boyer <jwboyer@fedoraproject.org>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <CA+5PVA5gE_pG5iPKBgugsEjLsOYvq9bWKfB+2OeEujjdOb+URg@mail.gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: netdev.vger.kernel.org

On Thu, Apr 23, 2015 at 10:07 AM, Josh Boyer <jwboyer@fedoraproject.org> wrote:
> [   29.382235] br0: port 1(tap0) entered forwarding state
>
> [   29.382286] ===============================
> [   29.382315] [ INFO: suspicious RCU usage. ]
> [   29.382344] 4.1.0-0.rc0.git11.1.fc23.x86_64 #1 Not tainted
> [   29.382380] -------------------------------
> [   29.382409] net/bridge/br_private.h:626 suspicious
> rcu_dereference_check() usage!
> [   29.382455]
>                other info that might help us debug this:
>
> [   29.382507]
>                rcu_scheduler_active = 1, debug_locks = 0
> [   29.382549] 2 locks held by swapper/0/0:
> [   29.382576]  #0:  (((&p->forward_delay_timer))){+.-...}, at:
> [<ffffffff81139f75>] call_timer_fn+0x5/0x4f0
> [   29.382660]  #1:  (&(&br->lock)->rlock){+.-...}, at:
> [<ffffffffa0450dc1>] br_forward_delay_timer_expired+0x31/0x140
> [bridge]
> [   29.382754]
>                stack backtrace:
> [   29.382787] CPU: 0 PID: 0 Comm: swapper/0 Not tainted
> 4.1.0-0.rc0.git11.1.fc23.x86_64 #1
> [   29.382838] Hardware name: LENOVO 422916G/LENOVO, BIOS A1KT53AUS 04/07/2015
> [   29.382882]  0000000000000000 3ebfc20364115825 ffff880666603c48
> ffffffff81892d4b
> [   29.382943]  0000000000000000 ffffffff81e124e0 ffff880666603c78
> ffffffff8110bcd7
> [   29.383004]  ffff8800785c9d00 ffff88065485ac58 ffff880c62002800
> ffff880c5fc88ac0
> [   29.383065] Call Trace:
> [   29.383084]  <IRQ>  [<ffffffff81892d4b>] dump_stack+0x4c/0x65
> [   29.383130]  [<ffffffff8110bcd7>] lockdep_rcu_suspicious+0xe7/0x120
> [   29.383178]  [<ffffffffa04520f9>] br_fill_ifinfo+0x4a9/0x6a0 [bridge]
> [   29.383225]  [<ffffffffa045266b>] br_ifinfo_notify+0x11b/0x4b0 [bridge]
> [   29.383271]  [<ffffffffa0450d90>] ? br_hold_timer_expired+0x70/0x70 [bridge]
> [   29.383320]  [<ffffffffa0450de8>]
> br_forward_delay_timer_expired+0x58/0x140 [bridge]
> [   29.383371]  [<ffffffffa0450d90>] ? br_hold_timer_expired+0x70/0x70 [bridge]
> [   29.383416]  [<ffffffff8113a033>] call_timer_fn+0xc3/0x4f0
> [   29.383454]  [<ffffffff81139f75>] ? call_timer_fn+0x5/0x4f0
> [   29.383493]  [<ffffffff8110a90f>] ? lock_release_holdtime.part.29+0xf/0x200
> [   29.383541]  [<ffffffffa0450d90>] ? br_hold_timer_expired+0x70/0x70 [bridge]
> [   29.383587]  [<ffffffff8113a6a4>] run_timer_softirq+0x244/0x490
> [   29.383629]  [<ffffffff810b68cc>] __do_softirq+0xec/0x670
> [   29.383666]  [<ffffffff810b70d5>] irq_exit+0x145/0x150
> [   29.383703]  [<ffffffff8189f506>] smp_apic_timer_interrupt+0x46/0x60
> [   29.383744]  [<ffffffff8189d523>] apic_timer_interrupt+0x73/0x80
> [   29.383782]  <EOI>  [<ffffffff816f131f>] ? cpuidle_enter_state+0x5f/0x2f0
> [   29.383832]  [<ffffffff816f131b>] ? cpuidle_enter_state+0x5b/0x2f0
> [   29.383873]  [<ffffffff816f15e7>] cpuidle_enter+0x17/0x20
> [   29.383908]  [<ffffffff81103c6f>] cpu_startup_entry+0x36f/0x5f0
> [   29.383949]  [<ffffffff81887cbd>] rest_init+0x13d/0x150
> [   29.383986]  [<ffffffff821c905b>] start_kernel+0x4d2/0x4f3
> [   29.384023]  [<ffffffff821c8120>] ? early_idt_handlers+0x120/0x120
> [   29.384064]  [<ffffffff821c8339>] x86_64_start_reservations+0x2a/0x2c
> [   29.384105]  [<ffffffff821c8485>] x86_64_start_kernel+0x14a/0x16d

We are dereferencing a RCU pointer with rtnl assert, but in the timer context
we only have br->lock. It looks like we need to take RCU read lock on that path
rather than asserting rtnl lock.

Thanks for the report.