Received-SPF: pass (google.com: domain of linux-wireless-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210717204057.67495-1-martin.blumenstingl@googlemail.com>
 <20210717204057.67495-6-martin.blumenstingl@googlemail.com> <1a299cd8c1be4fba8360780ef6f70f0f@realtek.com>
In-Reply-To: <1a299cd8c1be4fba8360780ef6f70f0f@realtek.com>
From:   Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Date:   Sun, 25 Jul 2021 23:36:13 +0200
Message-ID: <CAFBinCAJNqbpoqSSFYYBJg818KHCKx5nFzsKZdR=D+sTXQj6dg@mail.gmail.com>
Subject: Re: [PATCH RFC v1 5/7] rtw88: Configure the registers from
 rtw_bf_assoc() outside the RCU lock
To:     Pkshih <pkshih@realtek.com>
Cc:     "linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>,
        "tony0620emma@gmail.com" <tony0620emma@gmail.com>,
        "kvalo@codeaurora.org" <kvalo@codeaurora.org>,
        "johannes@sipsolutions.net" <johannes@sipsolutions.net>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Neo Jou <neojou@gmail.com>,
        Jernej Skrabec <jernej.skrabec@gmail.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

Hi Ping-Ke,

On Mon, Jul 19, 2021 at 7:47 AM Pkshih <pkshih@realtek.com> wrote:
[...]
> The rcu_read_lock() in this function is used to access ieee80211_find_sta() and protect 'sta'.
> A simple way is to shrink the critical section, like:
>
>         rcu_read_lock();
>
>         sta = ieee80211_find_sta(vif, bssid);
>         if (!sta) {
>                 rtw_warn(rtwdev, "failed to find station entry for bss %pM\n",
>                          bssid);
>                 rcu_read_unlock();
>         }
>
>         vht_cap = &sta->vht_cap;
>
>         rcu_read_unlock();
I agree that reducing the amount of code under the lock will help my
use-case as well
in your code-example I am wondering if we should change
  struct ieee80211_sta_vht_cap *vht_cap;
  vht_cap = &sta->vht_cap;
to
  struct ieee80211_sta_vht_cap vht_cap;
  vht_cap = sta->vht_cap;

My thinking is that ieee80211_sta may be freed in parallel to this code running.
If that cannot happen then your code will be fine.

So I am hoping that you can also share your thoughts on this one.


Thank you and best regards,
Martin