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Date:   Thu, 07 May 2020 07:58:04 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     mst@...hat.com
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        eric.dumazet@...il.com, jasowang@...hat.com,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH v2] virtio_net: fix lockdep warning on 32 bit

From: "Michael S. Tsirkin" <mst@...hat.com>
Date: Thu, 7 May 2020 03:25:56 -0400

> When we fill up a receive VQ, try_fill_recv currently tries to count
> kicks using a 64 bit stats counter. Turns out, on a 32 bit kernel that
> uses a seqcount. sequence counts are "lock" constructs where you need to
> make sure that writers are serialized.
> 
> In turn, this means that we mustn't run two try_fill_recv concurrently.
> Which of course we don't. We do run try_fill_recv sometimes from a
> softirq napi context, and sometimes from a fully preemptible context,
> but the later always runs with napi disabled.
> 
> However, when it comes to the seqcount, lockdep is trying to enforce the
> rule that the same lock isn't accessed from preemptible and softirq
> context - it doesn't know about napi being enabled/disabled. This causes
> a false-positive warning:
> 
> WARNING: inconsistent lock state
> ...
> inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
> 
> As a work around, shut down the warning by switching
> to u64_stats_update_begin_irqsave - that works by disabling
> interrupts on 32 bit only, is a NOP on 64 bit.
> 
> Reported-by: Thomas Gleixner <tglx@...utronix.de>
> Suggested-by: Eric Dumazet <eric.dumazet@...il.com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>

Applied and queued up for -stable, thanks Michael.

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