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From:   Kalle Valo <kvalo@kernel.org>
To:     Sascha Hauer <s.hauer@pengutronix.de>
Cc:     linux-wireless@vger.kernel.org, Neo Jou <neojou@gmail.com>,
        Hans Ulli Kroll <linux@ulli-kroll.de>,
        Ping-Ke Shih <pkshih@realtek.com>,
        Yan-Hsuan Chuang <tony0620emma@gmail.com>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Martin Blumenstingl <martin.blumenstingl@googlemail.com>,
        kernel@pengutronix.de, Johannes Berg <johannes@sipsolutions.net>
Subject: Re: [PATCH 00/10] RTW88: Add support for USB variants
References: <20220518082318.3898514-1-s.hauer@pengutronix.de>
        <87fskrv0cm.fsf@kernel.org> <20220530095232.GI1615@pengutronix.de>
Date:   Mon, 30 May 2022 13:07:25 +0300
In-Reply-To: <20220530095232.GI1615@pengutronix.de> (Sascha Hauer's message of
        "Mon, 30 May 2022 11:52:32 +0200")
Message-ID: <87a6azpc4i.fsf@kernel.org>
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Sascha Hauer <s.hauer@pengutronix.de> writes:

> On Mon, May 30, 2022 at 12:25:13PM +0300, Kalle Valo wrote:
>> Sascha Hauer <s.hauer@pengutronix.de> writes:
>> 
>> > Another problem to address is that the driver uses
>> > ieee80211_iterate_stations_atomic() and
>> > ieee80211_iterate_active_interfaces_atomic() and does register accesses
>> > in the iterator. This doesn't work with USB, so iteration is done in two
>> > steps now: The ieee80211_iterate_*_atomic() functions are only used to
>> > collect the stations/interfaces on a list which is then iterated over
>> > non-atomically in the second step. The implementation for this is
>> > basically the one suggested by Ping-Ke here:
>> >
>> > https://lore.kernel.org/lkml/423f474e15c948eda4db5bc9a50fd391@realtek.com/
>> 
>> Isn't this racy? What guarantees that vifs are not deleted after
>> ieee80211_iterate_active_interfaces_atomic() call?
>
> The driver mutex &rtwdev->mutex is acquired during the whole
> collection/iteration process. For deleting an interface
> ieee80211_ops::remove_interface would have to be called, right?
> That would acquire &rtwdev->mutex as well, so I think this should be
> safe.

Can you add a comment to the code explaining this? And
lockdep_assert_held() is a good way to guarantee that the mutex is
really held.

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