netdev - Re: [PATCH net v2] net: core: make napi

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Message-ID: <c72fd322-5181-16d6-5992-0fd71a083c31@huawei.com>
Date:   Wed, 14 Apr 2021 16:45:55 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Lijun Pan <lijunp213@...il.com>, <netdev@...r.kernel.org>
Subject: Re: [PATCH net v2] net: core: make napi_disable more robust

On 2021/4/14 16:08, Lijun Pan wrote:
> There are chances that napi_disable can be called twice by NIC driver.
> This could generate deadlock. For example,
> the first napi_disable will spin until NAPI_STATE_SCHED is cleared
> by napi_complete_done, then set it again.
> When napi_disable is called the second time, it will loop infinitely
> because no dev->poll will be running to clear NAPI_STATE_SCHED.
> 
> Though it is driver writer's responsibility to make sure it being
> called only once, making napi_disable more robust does not hurt, not
> to say it can prevent a buggy driver from crashing a system.
> So, we check the napi state bit to make sure that if napi is already
> disabled, we exit the call early enough to avoid spinning infinitely.
> 
> Fixes: bea3348eef27 ("[NET]: Make NAPI polling independent of struct net_device objects.")
> Signed-off-by: Lijun Pan <lijunp213@...il.com>
> ---
> v2: justify that this patch makes napi_disable more robust.
> 
>  net/core/dev.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 1f79b9aa9a3f..fa0aa212b7bb 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -6830,6 +6830,24 @@ EXPORT_SYMBOL(netif_napi_add);
>  void napi_disable(struct napi_struct *n)
>  {
>  	might_sleep();
> +
> +	/* make sure napi_disable() runs only once,
> +	 * When napi is disabled, the state bits are like:
> +	 * NAPI_STATE_SCHED (set by previous napi_disable)
> +	 * NAPI_STATE_NPSVC (set by previous napi_disable)
> +	 * NAPI_STATE_DISABLE (cleared by previous napi_disable)
> +	 * NAPI_STATE_PREFER_BUSY_POLL (cleared by previous napi_complete_done)
> +	 * NAPI_STATE_MISSED (cleared by previous napi_complete_done)
> +	 */
> +
> +	if (napi_disable_pending(n))
> +		return;
> +	if (test_bit(NAPI_STATE_SCHED, &n->state) &&
> +	    test_bit(NAPI_STATE_NPSVC, &n->state) &&
> +	    !test_bit(NAPI_STATE_MISSED, &n->state) &&
> +	    !test_bit(NAPI_STATE_PREFER_BUSY_POLL, &n->state))
> +		return;

The NAPI_STATE_DISABLE is cleared at the end of napi_disable(),
and if a buggy driver/hw triggers a interrupt and driver calls
napi_schedule_irqoff(), which may set NAPI_STATE_MISSED
if NAPI_STATE_SCHED is set(in napi_schedule_prep()), the above
checking does not seem to handle it?

> +
>  	set_bit(NAPI_STATE_DISABLE, &n->state);
>  
>  	while (test_and_set_bit(NAPI_STATE_SCHED, &n->state))
>