linux-kernel - RE: [PATCH RFC v1 3/7] rtw88: Use rtw_iterate_stas where the iterator reads or writes registers

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Message-ID: <981001e2981346ada4dcc08558b87a18@realtek.com>
Date:   Mon, 26 Jul 2021 07:22:52 +0000
From:   Pkshih <pkshih@...ltek.com>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Johannes Berg <johannes@...solutions.net>
CC:     "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "tony0620emma@...il.com" <tony0620emma@...il.com>,
        "kvalo@...eaurora.org" <kvalo@...eaurora.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Neo Jou <neojou@...il.com>,
        Jernej Skrabec <jernej.skrabec@...il.com>
Subject: RE: [PATCH RFC v1 3/7] rtw88: Use rtw_iterate_stas where the iterator reads or writes registers


> -----Original Message-----
> From: Martin Blumenstingl [mailto:martin.blumenstingl@...glemail.com]
> Sent: Monday, July 26, 2021 5:51 AM
> To: Johannes Berg; Pkshih
> Cc: linux-wireless@...r.kernel.org; tony0620emma@...il.com; kvalo@...eaurora.org;
> netdev@...r.kernel.org; linux-kernel@...r.kernel.org; Neo Jou; Jernej Skrabec
> Subject: Re: [PATCH RFC v1 3/7] rtw88: Use rtw_iterate_stas where the iterator reads or writes registers
> 
> Hi Johannes, Hi Ping-Ke,
> 
> On Mon, Jul 19, 2021 at 8:36 AM Johannes Berg <johannes@...solutions.net> wrote:
> >
> > On Sat, 2021-07-17 at 22:40 +0200, Martin Blumenstingl wrote:
> > >
> > > --- a/drivers/net/wireless/realtek/rtw88/mac80211.c
> > > +++ b/drivers/net/wireless/realtek/rtw88/mac80211.c
> > > @@ -721,7 +721,7 @@ static void rtw_ra_mask_info_update(struct rtw_dev *rtwdev,
> > >       br_data.rtwdev = rtwdev;
> > >       br_data.vif = vif;
> > >       br_data.mask = mask;
> > > -     rtw_iterate_stas_atomic(rtwdev, rtw_ra_mask_info_update_iter, &br_data);
> > > +     rtw_iterate_stas(rtwdev, rtw_ra_mask_info_update_iter, &br_data);
> >
> > And then you pretty much immediately break that invariant here, namely
> > that you're calling this within the set_bitrate_mask() method called by
> > mac80211.
> you are right, I was not aware of this
> 
> > That's not actually fundamentally broken today, but it does *severely*
> > restrict what we can do in mac80211 wrt. locking, and I really don't
> > want to keep the dozen or so locks forever, this needs simplification
> > because clearly we don't even know what should be under what lock.
> To me it's also not clear what the goal of the whole locking is.
> The lock in ieee80211_iterate_stations_atomic is obviously for the
> mac80211-internal state-machine
> But I *believe* that there's a second purpose (rtw88 specific) -
> here's my understanding of that part:
> - rtw_sta_info contains a "mac_id" which is an identifier for a
> specific station used by the rtw88 driver and is shared with the
> firmware
> - rtw_ops_sta_{add,remove} uses rtwdev->mutex to protect the rtw88
> side of this "mac_id" identifier
> - (for some reason rtw_update_sta_info doesn't use rtwdev->mutex)

I am thinking rtw88 needs to maintain sta and vif lists itself, and
these lists are also protected by rtwdev->mutex. When rtw88 wants to
iterate all sta/vif, it holds rtwdev->mutex to do list_for_each_entry.
No need to hold mac80211 locks.

> 
> So now I am wondering if the ieee80211_iterate_stations_atomic lock is
> also used to protect any modifications to rtw_sta_info.
> Ping-Ke, I am wondering if the attached patch (untested - to better
> demonstrate what I want to say) would:
> - allow us to move the register write outside of
> ieee80211_iterate_stations_atomic
> - mean we can keep ieee80211_iterate_stations_atomic (instead of the
> non-atomic variant)
> - protect the code managing the "mac_id" with rtwdev->mutex consistently

I think your attached patch can work well.

> 
> > The other cases look OK, it's being called from outside contexts
> > (wowlan, etc.)
> Thanks for reviewing this Johannes!
> 

--
Ping-Ke