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From:   Jassi Brar <jassisinghbrar@gmail.com>
Date:   Sun, 29 Dec 2019 21:28:05 -0600
Message-ID: <CABb+yY2w_h-weh15hFkGuC-b7XLYrvkT7QVkkj1u6uS99qOnrg@mail.gmail.com>
Subject: Re: [PATCH] soc: mediatek: cmdq: avoid racing condition with mutex
To:     CK Hu <ck.hu@mediatek.com>
Cc:     Bibby Hsieh <bibby.hsieh@mediatek.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Rob Herring <robh+dt@kernel.org>,
        Devicetree List <devicetree@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-mediatek@lists.infradead.org, srv_heupstream@mediatek.com,
        Nicolas Boichat <drinkcat@chromium.org>,
        Dennis-YC Hsieh <dennis-yc.hsieh@mediatek.com>,
        Houlong Wei <houlong.wei@mediatek.com>
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On Tue, Dec 3, 2019 at 8:22 PM CK Hu <ck.hu@mediatek.com> wrote:
>
> Hi, Jassi:
>
> Are mbox_send_message() and mbox_client_txdone() thread-safe? If these
> two are thread-safe, this bug should be fixed in mailbox core not
> client.
>
mbox_client_txdone should be called only when the client _knows_ the
message has been sent.
There is difference between knowing when tx is done, and assuming
tx-done because there is no way of knowing it.
Your issue arises because you immediately call mbox_client_txdone
after mbox_send_message, which may be the only way to do it but that
doesn't mean you shouldn't have to take any other precautions (like a
mutex). So I think your patch is reasonable.

Cheers!


> Regards,
> CK
>
> 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@mediatek.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);
> >
> >       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;
> >  };
> >
> >  /**
>