linux-kernel - RE: [PATCH net 1/6] net: hns3: fix side effects passed to min

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Message-ID: <85e3c423aa5a400981ae5c53a29ee280@AcuMS.aculab.com>
Date:   Fri, 28 Jul 2023 08:29:48 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jijie Shao' <shaojijie@...wei.com>,
        "yisen.zhuang@...wei.com" <yisen.zhuang@...wei.com>,
        "salil.mehta@...wei.com" <salil.mehta@...wei.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>
CC:     "shenjian15@...wei.com" <shenjian15@...wei.com>,
        "wangjie125@...wei.com" <wangjie125@...wei.com>,
        "liuyonglong@...wei.com" <liuyonglong@...wei.com>,
        "wangpeiyang1@...wei.com" <wangpeiyang1@...wei.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH net 1/6] net: hns3: fix side effects passed to min_t()

From: Jijie Shao
> Sent: 28 July 2023 08:59
> 
> num_online_cpus() may call more than once when passing to min_t(),
> between calls, it may return different values, so move num_online_cpus()
> out of min_t().

Nope, wrong bug:
min() (and friends) are careful to only evaluate their arguments once.
The bug is using min_t() - especially with a small type.

If/when the number of cpu hits 65536 the (u16) cast will convert
it to zero.

Looking at the code a lot of the local variables should be
'unsigned int' not 'u16.
Just because the domain of a value is small doesn't mean
you should use a small type (unless you are saving space in
a structure).

	David

> 
> Signed-off-by: Yonglong Liu <liuyonglong@...wei.com>
> Signed-off-by: Jijie Shao <shaojijie@...wei.com>
> ---
>  drivers/net/ethernet/hisilicon/hns3/hns3_enet.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c
> b/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c
> index 9f6890059666..823e6d2e85f5 100644
> --- a/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c
> +++ b/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c
> @@ -4757,6 +4757,7 @@ static int hns3_nic_alloc_vector_data(struct hns3_nic_priv *priv)
>  {
>  	struct hnae3_handle *h = priv->ae_handle;
>  	struct hns3_enet_tqp_vector *tqp_vector;
> +	u32 online_cpus = num_online_cpus();
>  	struct hnae3_vector_info *vector;
>  	struct pci_dev *pdev = h->pdev;
>  	u16 tqp_num = h->kinfo.num_tqps;
> @@ -4766,7 +4767,7 @@ static int hns3_nic_alloc_vector_data(struct hns3_nic_priv *priv)
> 
>  	/* RSS size, cpu online and vector_num should be the same */
>  	/* Should consider 2p/4p later */
> -	vector_num = min_t(u16, num_online_cpus(), tqp_num);
> +	vector_num = min_t(u16, online_cpus, tqp_num);
> 
>  	vector = devm_kcalloc(&pdev->dev, vector_num, sizeof(*vector),
>  			      GFP_KERNEL);
> --
> 2.30.0

-
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