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

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Date:   Mon, 16 May 2022 13:57:00 -0700
From:   "Martinez, Ricardo" <ricardo.martinez@...ux.intel.com>
To:     Sergey Ryazanov <ryazanov.s.a@...il.com>,
        Ziyang Xuan <william.xuanziyang@...wei.com>
Cc:     "Devegowda, Chandrashekar" <chandrashekar.devegowda@...el.com>,
        Intel Corporation <linuxwwan@...el.com>,
        chiranjeevi.rapolu@...ux.intel.com,
        Haijun Liu (刘海军) 
        <haijun.liu@...iatek.com>,
        M Chetan Kumar <m.chetan.kumar@...ux.intel.com>,
        Loic Poulain <loic.poulain@...aro.org>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: wwan: t7xx: fix GFP_KERNEL usage in
 spin_lock context


On 5/16/2022 1:36 PM, Sergey Ryazanov wrote:
> Hello Ziyang,
>
> On Sat, May 14, 2022 at 11:57 AM 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.
>>
>> Replace GFP_KERNEL with GFP_ATOMIC to fix it.
> Would not it will be more reliable to just rework
> t7xx_cldma_clear_rxq() to avoid calling t7xx_cldma_alloc_and_map_skb()
> under the spin lock instead of doing each allocation with GFP_ATOMIC?
> E.g. t7xx_cldma_gpd_rx_from_q() calls t7xx_cldma_alloc_and_map_skb()
> avoiding any lock holding.

t7xx_cldma_clear_rxq() is a helper for t7xx_cldma_clear_all_qs() which 
is only called by t7xx_cldma_exception() after stopping CLDMA, so it 
should be OK to remove the spin lock from t7xx_cldma_clear_rxq().