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Date: Wed, 06 May 2020 00:30:52 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: netdev@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [BUG] Inconsistent lock state in virtnet poll
"Michael S. Tsirkin" <mst@...hat.com> writes:
> On Tue, May 05, 2020 at 02:08:56PM +0200, Thomas Gleixner wrote:
>>
>> The following lockdep splat happens reproducibly on 5.7-rc4
>
>> ================================
>> WARNING: inconsistent lock state
>> 5.7.0-rc4+ #79 Not tainted
>> --------------------------------
>> inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
>> ip/356 [HC0[0]:SC1[1]:HE1:SE0] takes:
>> f3ee4cd8 (&syncp->seq#2){+.?.}-{0:0}, at: net_rx_action+0xfb/0x390
>> {SOFTIRQ-ON-W} state was registered at:
>> lock_acquire+0x82/0x300
>> try_fill_recv+0x39f/0x590
>
> Weird. Where does try_fill_recv acquire any locks?
u64_stats_update_begin(&rq->stats.syncp);
That's a 32bit kernel which uses a seqcount for this. sequence counts
are "lock" constructs where you need to make sure that writers are
serialized.
Actually the problem at hand is that try_fill_recv() is called from
fully preemptible context initialy and then from softirq context.
Obviously that's for the open() path a non issue, but lockdep does not
know about that. OTOH, there is other code which calls that from
non-softirq context.
The hack below made it shut up. It's obvioulsy not ideal, but at least
it let me look at the actual problem I was chasing down :)
Thanks,
tglx
8<-----------
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1243,9 +1243,11 @@ static bool try_fill_recv(struct virtnet
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;
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