linux-kernel - Re: [PATCH] soc: mediatek: cmdq: avoid racing condition with mutex

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Date:   Mon, 30 Dec 2019 14:44:13 +0800
From:   CK Hu <ck.hu@...iatek.com>
To:     Bibby Hsieh <bibby.hsieh@...iatek.com>
CC:     Jassi Brar <jassisinghbrar@...il.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Rob Herring <robh+dt@...nel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>,
        <srv_heupstream@...iatek.com>,
        Nicolas Boichat <drinkcat@...omium.org>,
        Dennis-YC Hsieh <dennis-yc.hsieh@...iatek.com>,
        Houlong Wei <houlong.wei@...iatek.com>
Subject: Re: [PATCH] soc: mediatek: cmdq: avoid racing condition with mutex

Hi, Bibby:

On Thu, 2019-11-21 at 15:29 +0800, Bibby Hsieh wrote:
> If cmdq client is multi thread user, racing will occur without mutex
> protection. It will make the C message queued in mailbox's queue
> always need D message's triggering.
> 
> Thread A		Thread B		  Thread C		Thread D...
> -----------------------------------------------------------------------------------
> mbox_send_message()
> 	send_data()
> 			mbox_send_message()
> 				*exit
> 						mbox_send_message()
> 							*exit
> mbox_client_txdone()
> 	tx_tick()
> 			mbox_client_txdone()
> 				tx_tick()
> 						mbox_client_txdone()
> 							tx_tick()
> msg_submit()
> 	send_data()
> 			msg_submit()
> 				*exit
> 						msg_submit()
> 							*exit
> -----------------------------------------------------------------------------------
> 
> Signed-off-by: Bibby Hsieh <bibby.hsieh@...iatek.com>
> ---
>  drivers/soc/mediatek/mtk-cmdq-helper.c | 3 +++
>  include/linux/soc/mediatek/mtk-cmdq.h  | 1 +
>  2 files changed, 4 insertions(+)
> 
> diff --git a/drivers/soc/mediatek/mtk-cmdq-helper.c b/drivers/soc/mediatek/mtk-cmdq-helper.c
> index 9add0fd5fa6c..9e35e0beffaa 100644
> --- a/drivers/soc/mediatek/mtk-cmdq-helper.c
> +++ b/drivers/soc/mediatek/mtk-cmdq-helper.c
> @@ -81,6 +81,7 @@ struct cmdq_client *cmdq_mbox_create(struct device *dev, int index, u32 timeout)
>  	client->client.dev = dev;
>  	client->client.tx_block = false;
>  	client->chan = mbox_request_channel(&client->client, index);
> +	mutex_init(&client->mutex);
>  
>  	if (IS_ERR(client->chan)) {
>  		long err;
> @@ -352,9 +353,11 @@ int cmdq_pkt_flush_async(struct cmdq_pkt *pkt, cmdq_async_flush_cb cb,
>  		spin_unlock_irqrestore(&client->lock, flags);
>  	}
>  
> +	mutex_lock(&client->mutex);
>  	mbox_send_message(client->chan, pkt);
>  	/* We can send next packet immediately, so just call txdone. */
>  	mbox_client_txdone(client->chan, 0);
> +	mutex_unlock(&client->mutex);


In [1], Mediatek DRM is the first client to use cmdq and it already has
its own mutex to protect this. I think helper is something help many
user but now I just see Mediatek MDP [2] need this. For DRM, there are
so many useless code in cmdq_pkt_flush_async(). DRM does not need the
timer to check timeout. DRM could do dma_sync_single_for_cpu() in its
callback and need not to create a intermediate callback to do this. I
would agree this patch only when I see two or more user need this.


[1]
https://github.com/ckhu-mediatek/linux.git-tags/commit/4df12ed1866d1104f631e06218bd15fde512a79e
[2] https://patchwork.kernel.org/patch/10945609/

Regards,
CK

>  
>  	return 0;
>  }
> diff --git a/include/linux/soc/mediatek/mtk-cmdq.h b/include/linux/soc/mediatek/mtk-cmdq.h
> index a74c1d5acdf3..0f9071cd1bc7 100644
> --- a/include/linux/soc/mediatek/mtk-cmdq.h
> +++ b/include/linux/soc/mediatek/mtk-cmdq.h
> @@ -28,6 +28,7 @@ struct cmdq_client {
>  	struct mbox_chan *chan;
>  	struct timer_list timer;
>  	u32 timeout_ms; /* in unit of microsecond */
> +	struct mutex mutex;
>  };
>  
>  /**