netdev - Re: [RFC] virtio_net: add local_bh_disable() around u64_stats_update

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Date:   Wed, 17 Oct 2018 14:48:02 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org
Cc:     tglx@...utronix.de, "Michael S. Tsirkin" <mst@...hat.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC] virtio_net: add local_bh_disable() around
 u64_stats_update_begin


On 2018/10/17 上午9:13, Toshiaki Makita wrote:
> On 2018/10/17 1:55, Sebastian Andrzej Siewior wrote:
>> on 32bit, lockdep notices:
>> | ================================
>> | WARNING: inconsistent lock state
>> | 4.19.0-rc8+ #9 Tainted: G        W
>> | --------------------------------
>> | inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
>> | ip/1106 [HC0[0]:SC1[1]:HE1:SE0] takes:
>> | (ptrval) (&syncp->seq#2){+.?.}, at: net_rx_action+0xc8/0x380
>> | {SOFTIRQ-ON-W} state was registered at:
>> |   lock_acquire+0x7e/0x170
>> |   try_fill_recv+0x5fa/0x700
>> |   virtnet_open+0xe0/0x180
>> |   __dev_open+0xae/0x130
>> |   __dev_change_flags+0x17f/0x200
>> |   dev_change_flags+0x23/0x60
>> |   do_setlink+0x2bb/0xa20
>> |   rtnl_newlink+0x523/0x830
>> |   rtnetlink_rcv_msg+0x14b/0x470
>> |   netlink_rcv_skb+0x6e/0xf0
>> |   rtnetlink_rcv+0xd/0x10
>> |   netlink_unicast+0x16e/0x1f0
>> |   netlink_sendmsg+0x1af/0x3a0
>> |   ___sys_sendmsg+0x20f/0x240
>> |   __sys_sendmsg+0x39/0x80
>> |   sys_socketcall+0x13a/0x2a0
>> |   do_int80_syscall_32+0x50/0x180
>> |   restore_all+0x0/0xb2
>> | irq event stamp: 3326
>> | hardirqs last  enabled at (3326): [<c159e6d0>] net_rx_action+0x80/0x380
>> | hardirqs last disabled at (3325): [<c159e6aa>] net_rx_action+0x5a/0x380
>> | softirqs last  enabled at (3322): [<c14b440d>] virtnet_napi_enable+0xd/0x60
>> | softirqs last disabled at (3323): [<c101d63d>] call_on_stack+0xd/0x50
>> |
>> | other info that might help us debug this:
>> |  Possible unsafe locking scenario:
>> |
>> |        CPU0
>> |        ----
>> |   lock(&syncp->seq#2);
>> |   <Interrupt>
>> |     lock(&syncp->seq#2);
>> |
>> |  *** DEADLOCK ***
> IIUC try_fill_recv is called only when NAPI is disabled from process
> context, so there should be no point to race with virtnet_receive which
> is called from NAPI handler.
>
> I'm not sure what condition triggered this warning.
>
>
> Toshiaki Makita


Or maybe NAPI is enabled unexpectedly somewhere?

Btw, the schedule_delayed_work() in virtnet_open() is also suspicious, 
if the work is executed before virtnet_napi_enable(), there will be a 
deadloop for napi_disable().

Thanks


>
>
>> This is the "up" path which is not a hotpath. There is also
>> refill_work().
>> It might be unwise to add the local_bh_disable() to try_fill_recv()
>> because if it is used mostly in BH so that local_bh_en+dis might be a
>> waste of cycles.
>>
>> Adding local_bh_disable() around try_fill_recv() for the non-BH call
>> sites would render GFP_KERNEL pointless.
>>
>> Also, ptr->var++ is not an atomic operation even on 64bit CPUs. Which
>> means if try_fill_recv() runs on CPU0 (via virtnet_receive()) then the
>> worker might run on CPU1.
>>
>> Do we care or is this just stupid stats?  Any suggestions?
>>
>> This warning appears since commit 461f03dc99cf6 ("virtio_net: Add kick stats").
>>
>> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
>> ---
>>   drivers/net/virtio_net.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index dab504ec5e502..d782160cfa882 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -1206,9 +1206,11 @@ static bool try_fill_recv(struct virtnet_info *vi, struct receive_queue *rq,
>>   			break;
>>   	} while (rq->vq->num_free);
>>   	if (virtqueue_kick_prepare(rq->vq) && virtqueue_notify(rq->vq)) {
>> +		local_bh_disable();
>>   		u64_stats_update_begin(&rq->stats.syncp);
>>   		rq->stats.kicks++;
>>   		u64_stats_update_end(&rq->stats.syncp);
>> +		local_bh_enable();
>>   	}
>>   
>>   	return !oom;
>>