[PATCH net-next] net: warn if NAPI instance wasn't shut down

[PATCH net-next] net: warn if NAPI instance wasn't shut down

[Jakub Kicinski]

Drivers should always disable a NAPI instance before removing it.
If they don't the instance may be queued for polling.
Since commit 86e25f40aa1e ("net: napi: Add napi_config")
we also remove the NAPI from the busy polling hash table
in napi_disable(), so not disabling would leave a stale
entry there.

Use of busy polling is relatively uncommon so bugs may be lurking
in the drivers. Add an explicit warning.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 net/core/dev.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index c0021cbd28fc..2b141f20b13b 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -7071,6 +7071,9 @@ void __netif_napi_del_locked(struct napi_struct *napi)
 	if (!test_and_clear_bit(NAPI_STATE_LISTED, &napi->state))
 		return;
 
+	/* Make sure NAPI is disabled (or was never enabled). */
+	WARN_ON(!test_bit(NAPI_STATE_SCHED, &napi->state));
+
 	if (napi->config) {
 		napi->index = -1;
 		napi->config = NULL;
-- 
2.48.1

Re: [PATCH net-next] net: warn if NAPI instance wasn't shut down

[Joe Damato]

On Mon, Feb 03, 2025 at 01:58:16PM -0800, Jakub Kicinski wrote:
> Drivers should always disable a NAPI instance before removing it.
> If they don't the instance may be queued for polling.
> Since commit 86e25f40aa1e ("net: napi: Add napi_config")
> we also remove the NAPI from the busy polling hash table
> in napi_disable(), so not disabling would leave a stale
> entry there.
> 
> Use of busy polling is relatively uncommon so bugs may be lurking
> in the drivers. Add an explicit warning.
> 
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> ---
>  net/core/dev.c | 3 +++
>  1 file changed, 3 insertions(+)
> 

Let's find out how many bugs are lurking:

Reviewed-by: Joe Damato <jdamato@fastly.com>

Re: [PATCH net-next] net: warn if NAPI instance wasn't shut down

[Eric Dumazet]

On Mon, Feb 3, 2025 at 10:58 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> Drivers should always disable a NAPI instance before removing it.
> If they don't the instance may be queued for polling.
> Since commit 86e25f40aa1e ("net: napi: Add napi_config")
> we also remove the NAPI from the busy polling hash table
> in napi_disable(), so not disabling would leave a stale
> entry there.
>
> Use of busy polling is relatively uncommon so bugs may be lurking
> in the drivers. Add an explicit warning.
>
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> ---
>  net/core/dev.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index c0021cbd28fc..2b141f20b13b 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -7071,6 +7071,9 @@ void __netif_napi_del_locked(struct napi_struct *napi)
>         if (!test_and_clear_bit(NAPI_STATE_LISTED, &napi->state))
>                 return;
>
> +       /* Make sure NAPI is disabled (or was never enabled). */
> +       WARN_ON(!test_bit(NAPI_STATE_SCHED, &napi->state));
> +
>         if (napi->config) {
>                 napi->index = -1;
>                 napi->config = NULL;
> --
> 2.48.1
>

This makes sense. Although a WARN_ON_ONCE() might avoid some noise.

Reviewed-by: Eric Dumazet <edumazet@google.com>

Re: [PATCH net-next] net: warn if NAPI instance wasn't shut down

[patchwork-bot+netdevbpf]

Hello:

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

On Mon,  3 Feb 2025 13:58:16 -0800 you wrote:
> Drivers should always disable a NAPI instance before removing it.
> If they don't the instance may be queued for polling.
> Since commit 86e25f40aa1e ("net: napi: Add napi_config")
> we also remove the NAPI from the busy polling hash table
> in napi_disable(), so not disabling would leave a stale
> entry there.
> 
> [...]

Here is the summary with links:
  - [net-next] net: warn if NAPI instance wasn't shut down
    https://git.kernel.org/netdev/net-next/c/9dd05df8403b

You are awesome, thank you!
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