[PATCH net-next] net: avoid quadratic behavior in netdev_wait_allrefs_any()

[PATCH net-next] net: avoid quadratic behavior in netdev_wait_allrefs_any()

[Eric Dumazet]

From: Eric Dumazet <edumazet@google.com>

If the list of devices has N elements, netdev_wait_allrefs_any()
is called N times, and linkwatch_forget_dev() is called N*(N-1)/2 times.

Fix this by calling linkwatch_forget_dev() only once per device.

Fixes: faab39f63c1f ("net: allow out-of-order netdev unregistration")
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/core/dev.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 05fa867f18787e709dcaccfea1df350c424eff80..acd884910e12a040841e1e0525e0d4bc5e3ee799 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -9828,9 +9828,6 @@ static struct net_device *netdev_wait_allrefs_any(struct list_head *list)
 	struct net_device *dev;
 	int wait = 0;
 
-	list_for_each_entry(dev, list, todo_list)
-		linkwatch_forget_dev(dev);
-
 	rebroadcast_time = warning_time = jiffies;
 
 	list_for_each_entry(dev, list, todo_list)
@@ -9951,6 +9948,7 @@ void netdev_run_todo(void)
 		}
 
 		dev->reg_state = NETREG_UNREGISTERED;
+		linkwatch_forget_dev(dev);
 	}
 
 	while (!list_empty(&list)) {
-- 
2.35.1.265.g69c8d7142f-goog

Re: [PATCH net-next] net: avoid quadratic behavior in netdev_wait_allrefs_any()

[Jakub Kicinski]

On Thu, 17 Feb 2022 22:54:30 -0800 Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@google.com>
> 
> If the list of devices has N elements, netdev_wait_allrefs_any()
> is called N times, and linkwatch_forget_dev() is called N*(N-1)/2 times.
> 
> Fix this by calling linkwatch_forget_dev() only once per device.
> 
> Fixes: faab39f63c1f ("net: allow out-of-order netdev unregistration")
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Yup, I was on the fence on this one, I went for keeping all linkwatch
stuff in one function and minimal changes. Clearly underestimated the
number of netdevs your cases are destroying at once :)

Re: [PATCH net-next] net: avoid quadratic behavior in netdev_wait_allrefs_any()

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (master)
by Jakub Kicinski <kuba@kernel.org>:

On Thu, 17 Feb 2022 22:54:30 -0800 you wrote:
> From: Eric Dumazet <edumazet@google.com>
> 
> If the list of devices has N elements, netdev_wait_allrefs_any()
> is called N times, and linkwatch_forget_dev() is called N*(N-1)/2 times.
> 
> Fix this by calling linkwatch_forget_dev() only once per device.
> 
> [...]

Here is the summary with links:
  - [net-next] net: avoid quadratic behavior in netdev_wait_allrefs_any()
    https://git.kernel.org/netdev/net-next/c/86213f80da1b

You are awesome, thank you!
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