linux-kernel - Re: [PATCH v4 2/4] bus: mhi: host: Drop chan lock before queuing buffers

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Message-ID: <20231124100403.GA4536@thinkpad>
Date:   Fri, 24 Nov 2023 15:34:03 +0530
From:   Manivannan Sadhasivam <mani@...nel.org>
To:     Qiang Yu <quic_qianyu@...cinc.com>
Cc:     quic_jhugo@...cinc.com, mhi@...ts.linux.dev,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        quic_cang@...cinc.com, quic_mrana@...cinc.com
Subject: Re: [PATCH v4 2/4] bus: mhi: host: Drop chan lock before queuing
 buffers

On Tue, Nov 14, 2023 at 01:27:39PM +0800, Qiang Yu wrote:
> Ensure read and write locks for the channel are not taken in succession by
> dropping the read lock from parse_xfer_event() such that a callback given
> to client can potentially queue buffers and acquire the write lock in that
> process. Any queueing of buffers should be done without channel read lock
> acquired as it can result in multiple locks and a soft lockup.
> 

Is this patch trying to fix an existing issue in client drivers or a potential
issue in the future drivers?

Even if you take care of disabled channels, "mhi_event->lock" acquired during
mhi_mark_stale_events() can cause deadlock, since event lock is already held by
mhi_ev_task().

I'd prefer not to open the window unless this patch is fixing a real issue.

- Mani

> Signed-off-by: Qiang Yu <quic_qianyu@...cinc.com>
> ---
>  drivers/bus/mhi/host/main.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/bus/mhi/host/main.c b/drivers/bus/mhi/host/main.c
> index 6c6d253..c4215b0 100644
> --- a/drivers/bus/mhi/host/main.c
> +++ b/drivers/bus/mhi/host/main.c
> @@ -642,6 +642,8 @@ static int parse_xfer_event(struct mhi_controller *mhi_cntrl,
>  			mhi_del_ring_element(mhi_cntrl, tre_ring);
>  			local_rp = tre_ring->rp;
>  
> +			read_unlock_bh(&mhi_chan->lock);
> +
>  			/* notify client */
>  			mhi_chan->xfer_cb(mhi_chan->mhi_dev, &result);
>  
> @@ -667,6 +669,8 @@ static int parse_xfer_event(struct mhi_controller *mhi_cntrl,
>  					kfree(buf_info->cb_buf);
>  				}
>  			}
> +
> +			read_lock_bh(&mhi_chan->lock);
>  		}
>  		break;
>  	} /* CC_EOT */
> -- 
> 2.7.4
> 
> 

-- 
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