[PATCH v2 net 2/2] drop_monitor: perform u64_stats updates under IRQ-disabled section

[Eric Dumazet <edumazet@google.com>]

In net_dm_packet_trace_kfree_skb_hit() and net_dm_hw_trap_packet_probe(),
u64_stats_update_begin() / u64_stats_inc() / u64_stats_update_end() were
called after spin_unlock_irqrestore(&...drop_queue.lock, flags), when local
IRQs had already been re-enabled.

Tracepoint probes can execute in IRQ or softirq context. On 32-bit
architectures, u64_stats_update_begin() disables preemption but not interrupts,
relying on seqcount writes. If a nested interrupt occurs on the same CPU during
the 64-bit stats update, the reentrant seqcount update can corrupt the
seqcount state or stats value.

Fix this by performing the 64-bit per-CPU stats update before releasing
drop_queue.lock via spin_unlock_irqrestore(), ensuring local interrupts remain
disabled during the u64_stats update.

Fixes: e9feb58020f9 ("drop_monitor: Expose tail drop counter")
Fixes: 5e58109b1ea4 ("drop_monitor: Add support for packet alert mode for hardware drops")
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/core/drop_monitor.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/drop_monitor.c b/net/core/drop_monitor.c
index 5d63cf2c230b4368e8a84e52413d37da26fb279a..8800a21ded084025bd1a4e5365de9971ab660808 100644
--- a/net/core/drop_monitor.c
+++ b/net/core/drop_monitor.c
@@ -530,10 +530,10 @@ static void net_dm_packet_trace_kfree_skb_hit(void *ignore,
 	return;
 
 unlock_free:
-	spin_unlock_irqrestore(&data->drop_queue.lock, flags);
 	u64_stats_update_begin(&data->stats.syncp);
 	u64_stats_inc(&data->stats.dropped);
 	u64_stats_update_end(&data->stats.syncp);
+	spin_unlock_irqrestore(&data->drop_queue.lock, flags);
 	consume_skb(nskb);
 }
 
@@ -1001,10 +1001,10 @@ net_dm_hw_trap_packet_probe(void *ignore, const struct devlink *devlink,
 	return;
 
 unlock_free:
-	spin_unlock_irqrestore(&hw_data->drop_queue.lock, flags);
 	u64_stats_update_begin(&hw_data->stats.syncp);
 	u64_stats_inc(&hw_data->stats.dropped);
 	u64_stats_update_end(&hw_data->stats.syncp);
+	spin_unlock_irqrestore(&hw_data->drop_queue.lock, flags);
 	net_dm_hw_metadata_free(n_hw_metadata);
 free:
 	consume_skb(nskb);
-- 
2.55.0.229.g6434b31f56-goog