netdev - Re: [PATCH net v2] net: fix hangup on napi

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Date:   Fri, 9 Apr 2021 17:35:23 +0200
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Wei Wang <weiwan@...gle.com>
Subject: Re: [PATCH net v2] net: fix hangup on napi_disable for threaded napi

On 09.04.2021 17:24, Paolo Abeni wrote:
> napi_disable() is subject to an hangup, when the threaded
> mode is enabled and the napi is under heavy traffic.
> 
> If the relevant napi has been scheduled and the napi_disable()
> kicks in before the next napi_threaded_wait() completes - so
> that the latter quits due to the napi_disable_pending() condition,
> the existing code leaves the NAPI_STATE_SCHED bit set and the
> napi_disable() loop waiting for such bit will hang.
> 
> This patch addresses the issue by dropping the NAPI_STATE_DISABLE
> bit test in napi_thread_wait(). The later napi_threaded_poll()
> iteration will take care of clearing the NAPI_STATE_SCHED.
> 
> This also addresses a related problem reported by Jakub:
> before this patch a napi_disable()/napi_enable() pair killed
> the napi thread, effectively disabling the threaded mode.
> On the patched kernel napi_disable() simply stops scheduling
> the relevant thread.
> 
> v1 -> v2:
>   - let the main napi_thread_poll() loop clear the SCHED bit
> 
> Reported-by: Jakub Kicinski <kuba@...nel.org>
> Fixes: 29863d41bb6e ("net: implement threaded-able napi poll loop support")
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
>  net/core/dev.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 0f72ff5d34ba..af8c1ea040b9 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -6992,7 +6992,7 @@ static int napi_thread_wait(struct napi_struct *napi)
>  
>  	set_current_state(TASK_INTERRUPTIBLE);
>  
> -	while (!kthread_should_stop() && !napi_disable_pending(napi)) {
> +	while (!kthread_should_stop()) {
>  		/* Testing SCHED_THREADED bit here to make sure the current
>  		 * kthread owns this napi and could poll on this napi.
>  		 * Testing SCHED bit is not enough because SCHED bit might be
> @@ -7010,6 +7010,7 @@ static int napi_thread_wait(struct napi_struct *napi)
>  		set_current_state(TASK_INTERRUPTIBLE);
>  	}
>  	__set_current_state(TASK_RUNNING);
> +
>  	return -1;
>  }
>  
> 

Unrelated to the actual issue:
I wonder why -1 is returned, that's unusual in kernel. The return value
is used as bool, so why not return a bool?