[PATCH net v3] openvswitch: fix soft lockup in the netlink flow dump

["Denis V. Lunev" <den@openvz.org>]

From: Denis V. Lunev <den@openvz.org>

A production compute node carrying a few thousand datapath flows hit a
soft lockup inside a single netlink flow dump and panicked.

ovs_flow_cmd_dump() calls ovs_flow_stats_get() for every flow it
emits, and that releases stats->lock with spin_unlock_bh() once per
CPU that has touched the flow. Every release is a local_bh_enable(),
and each one runs the pending softirq backlog in the dumping thread's
own context.

The skb bounds the flows one callback emits, but not the softirq work
it absorbs. On a CPU that carries the box's packet load the backlog
refills as fast as it drains, so the dumping thread becomes that CPU's
softirq engine. It never sleeps and it has no reschedule point, so
under voluntary preemption nothing can take the CPU away from it:
neither the ksoftirqd the kernel woke to take the work over, nor the
stopper thread the softlockup detector dispatches to refresh its
timestamp.

Hold BH off across the whole callback instead, the way
ctnetlink_dump_table() does, so the nested spin_unlock_bh() stop
draining softirqs. The loop already runs under rcu_read_lock() and
cannot sleep. What it gives up is preemption under CONFIG_PREEMPT,
since a BH-off region is not preemptible outside PREEMPT_RT. The
region stays short: the skb caps the flows one callback emits, and
empty buckets cost no skb space but are each visited once per dump, as
the cursor only moves forward. The table grows on insert and shrinks
only on flush, so the walk is bounded by the largest flow count the
datapath has held. The softirq backlog the callback used to absorb has
no bound at all.

Fixes: 63e7959c4b9b ("openvswitch: Per NUMA node flow stats.")
Cc: stable@vger.kernel.org
Signed-off-by: Denis V. Lunev <den@openvz.org>
---
v3:
- add the net prefix, Fixes tag and Cc stable
- explain why the BH-off walk stays bounded: the cursor visits each
  empty bucket once per dump and the table shrinks only on flush
- drop "here" from the comment, add blank lines around the
  local_bh_disable()/local_bh_enable() pair
v2: https://lore.kernel.org/netdev/20260915122401.3910188-1-den@openvz.org/
- leave ovs_vport_cmd_dump() alone: nsid_lock has not been BH-safe
  since commit aed4969f2bdf ("net: net->nsid_lock does not need BH
  safety"), so the vport dump never drained softirqs
- disable BH before the table dereference and say in a comment that
  the region is not there for safety
- drop the ovs_flow_stats_get() history, note the empty-bucket walk
  and the lost CONFIG_PREEMPT preemption in the message
- move the Cc list out of the commit message
v1: https://lore.kernel.org/netdev/20260909171920.1001074-1-den@openvz.org/

 net/openvswitch/datapath.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
index 631a03136fa1..4fc5d0bebd85 100644
--- a/net/openvswitch/datapath.c
+++ b/net/openvswitch/datapath.c
@@ -1532,6 +1532,12 @@ static int ovs_flow_cmd_dump(struct sk_buff *skb, struct netlink_callback *cb)
 		return -ENODEV;
 	}
 
+	/*
+	 * Not needed for safety. Stops every spin_unlock_bh() in
+	 * ovs_flow_stats_get() from running the softirq backlog.
+	 */
+	local_bh_disable();
+
 	ti = rcu_dereference(dp->table.ti);
 	for (;;) {
 		struct sw_flow *flow;
@@ -1552,6 +1558,8 @@ static int ovs_flow_cmd_dump(struct sk_buff *skb, struct netlink_callback *cb)
 		cb->args[0] = bucket;
 		cb->args[1] = obj;
 	}
+
+	local_bh_enable();
 	rcu_read_unlock();
 	return skb->len;
 }
-- 
2.53.0