netdev - Re: [PATCH net-next v2] net: wwan: t7xx: fix GFP_KERNEL usage in spin

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Date:   Tue, 17 May 2022 10:50:51 +0200
From:   Loic Poulain <loic.poulain@...aro.org>
To:     Ziyang Xuan <william.xuanziyang@...wei.com>
Cc:     chandrashekar.devegowda@...el.com, linuxwwan@...el.com,
        chiranjeevi.rapolu@...ux.intel.com, haijun.liu@...iatek.com,
        m.chetan.kumar@...ux.intel.com, ricardo.martinez@...ux.intel.com,
        ryazanov.s.a@...il.com, johannes@...solutions.net,
        davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2] net: wwan: t7xx: fix GFP_KERNEL usage in
 spin_lock context

Hi Ziyang,

On Tue, 17 May 2022 at 08:30, Ziyang Xuan <william.xuanziyang@...wei.com> wrote:
>
> t7xx_cldma_clear_rxq() call t7xx_cldma_alloc_and_map_skb() in spin_lock
> context, But __dev_alloc_skb() in t7xx_cldma_alloc_and_map_skb() uses
> GFP_KERNEL, that will introduce scheduling factor in spin_lock context.
>
> Because t7xx_cldma_clear_rxq() is called after stopping CLDMA, so we can
> remove the spin_lock from t7xx_cldma_clear_rxq().
>
> Fixes: 39d439047f1d ("net: wwan: t7xx: Add control DMA interface")
> Signed-off-by: Ziyang Xuan <william.xuanziyang@...wei.com>
> ---

You should normally indicate what changed in this v2.

>  drivers/net/wwan/t7xx/t7xx_hif_cldma.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/wwan/t7xx/t7xx_hif_cldma.c b/drivers/net/wwan/t7xx/t7xx_hif_cldma.c
> index 46066dcd2607..7493285a9606 100644
> --- a/drivers/net/wwan/t7xx/t7xx_hif_cldma.c
> +++ b/drivers/net/wwan/t7xx/t7xx_hif_cldma.c
> @@ -782,10 +782,12 @@ static int t7xx_cldma_clear_rxq(struct cldma_ctrl *md_ctrl, int qnum)
>         struct cldma_queue *rxq = &md_ctrl->rxq[qnum];
>         struct cldma_request *req;
>         struct cldma_gpd *gpd;
> -       unsigned long flags;
>         int ret = 0;
>
> -       spin_lock_irqsave(&rxq->ring_lock, flags);
> +       /* CLDMA has been stopped. There is not any CLDMA IRQ, holding
> +        * ring_lock is not needed.

If it makes sense to explain why we don't need locking, the next
sentence is not needed:


>  Thus we can use functions that may
> +        * introduce scheduling.
> +        */
>         t7xx_cldma_q_reset(rxq);
>         list_for_each_entry(req, &rxq->tr_ring->gpd_ring, entry) {
>                 gpd = req->gpd;
> @@ -808,7 +810,6 @@ static int t7xx_cldma_clear_rxq(struct cldma_ctrl *md_ctrl, int qnum)
>
>                 t7xx_cldma_gpd_set_data_ptr(req->gpd, req->mapped_buff);
>         }
> -       spin_unlock_irqrestore(&rxq->ring_lock, flags);
>
>         return ret;
>  }
> --
> 2.25.1
>