From:   Kalle Valo <kvalo@kernel.org>
To:     Lorenzo Bianconi <lorenzo@kernel.org>
Cc:     nbd@nbd.name, lorenzo.bianconi@redhat.com,
        linux-wireless@vger.kernel.org, sean.wang@mediatek.com,
        deren.wu@mediatek.com
Subject: Re: [PATCH 1/2] mt76: mt7921: fix a leftover race in runtime-pm
References: <cover.1640897147.git.lorenzo@kernel.org>
        <65e65daddbcec420392befa3b4f9a6aaaea21315.1640897147.git.lorenzo@kernel.org>
Date:   Tue, 11 Jan 2022 12:35:13 +0200
In-Reply-To: <65e65daddbcec420392befa3b4f9a6aaaea21315.1640897147.git.lorenzo@kernel.org>
        (Lorenzo Bianconi's message of "Thu, 30 Dec 2021 21:47:05 +0100")
Message-ID: <87zgo2k1b2.fsf@tynnyri.adurom.net>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)
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Precedence: bulk

Lorenzo Bianconi <lorenzo@kernel.org> writes:

> Fix a possible race in mt7921_pm_power_save_work() if rx/tx napi
> schedules ps_work and we are currently accessing device register
> on a different cpu.
>
> Fixes: 1d8efc741df8 ("mt76: mt7921: introduce Runtime PM support")
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> ---
>  drivers/net/wireless/mediatek/mt76/mt7921/mac.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/drivers/net/wireless/mediatek/mt76/mt7921/mac.c b/drivers/net/wireless/mediatek/mt76/mt7921/mac.c
> index defef3496246..0744f6e42ba3 100644
> --- a/drivers/net/wireless/mediatek/mt76/mt7921/mac.c
> +++ b/drivers/net/wireless/mediatek/mt76/mt7921/mac.c
> @@ -1553,6 +1553,14 @@ void mt7921_pm_power_save_work(struct work_struct *work)
>  	    dev->fw_assert)
>  		goto out;
>  
> +	if (mutex_is_locked(&dev->mt76.mutex))
> +		/* if mt76 mutex is held we should not put the device
> +		 * to sleep since we are currently accessing device
> +		 * register map. We need to wait for the next power_save
> +		 * trigger.
> +		 */
> +		goto out;

This looks fishy to me. What protects the case when ps_work is run first
and at the same time another cpu starts accessing the registers?

Do note that I didn't check the code, so I might be missing something.

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