Michael S. Tsirkin
2020-May-06 00:01 UTC
[PATCH] virtio_net: fix lockdep warning on 32 bit
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 fully
preemptible context and sometimes from a softirq (napi) context.
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. 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 at linutronix.de>
Suggested-by: Eric Dumazet <eric.dumazet at gmail.com>
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
---
I'm not thrilled about this but this seems the best we can do for now.
Completely untested.
Thomas, can you pls let me know the config I need to trigger the warning
in question?
drivers/net/virtio_net.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 6594aab4910e..95393b61187f 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1243,9 +1243,11 @@ 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))
{
- u64_stats_update_begin(&rq->stats.syncp);
+ unsigned long flags;
+
+ flags = u64_stats_update_begin_irqsave(&rq->stats.syncp);
rq->stats.kicks++;
- u64_stats_update_end(&rq->stats.syncp);
+ u64_stats_update_end_irqrestore(&rq->stats.syncp);
}
return !oom;
--
MST
On 2020/5/6 ??8:01, Michael S. Tsirkin wrote:> 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 fully > preemptible context and sometimes from a softirq (napi) context. > > 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. 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 at linutronix.de> > Suggested-by: Eric Dumazet <eric.dumazet at gmail.com> > Signed-off-by: Michael S. Tsirkin <mst at redhat.com> > --- > > I'm not thrilled about this but this seems the best we can do for now. > > Completely untested. > > > Thomas, can you pls let me know the config I need to trigger the warning > in question? > > > drivers/net/virtio_net.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c > index 6594aab4910e..95393b61187f 100644 > --- a/drivers/net/virtio_net.c > +++ b/drivers/net/virtio_net.c > @@ -1243,9 +1243,11 @@ 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)) { > - u64_stats_update_begin(&rq->stats.syncp); > + unsigned long flags; > + > + flags = u64_stats_update_begin_irqsave(&rq->stats.syncp); > rq->stats.kicks++; > - u64_stats_update_end(&rq->stats.syncp); > + u64_stats_update_end_irqrestore(&rq->stats.syncp); > } > > return !oom;Acked-by: Jason Wang <jasowang at redhat.com>
From: "Michael S. Tsirkin" <mst at redhat.com> Date: Tue, 5 May 2020 20:01:31 -0400> - u64_stats_update_end(&rq->stats.syncp); > + u64_stats_update_end_irqrestore(&rq->stats.syncp);Need to pass flags to this function.