linux-kernel - Re: [PATCH v2 1/2] bus: mhi: host: Add spinlock to protect WP access when queueing TREs

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Date:   Fri, 22 Sep 2023 08:44:03 -0600
From:   Jeffrey Hugo <quic_jhugo@...cinc.com>
To:     Qiang Yu <quic_qianyu@...cinc.com>, <mani@...nel.org>
CC:     <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 v2 1/2] bus: mhi: host: Add spinlock to protect WP access
 when queueing TREs

On 9/13/2023 2:47 AM, Qiang Yu wrote:
> From: Bhaumik Bhatt <bbhatt@...eaurora.org>
> 
> Protect WP accesses such that multiple threads queueing buffers for
> incoming data do not race and access the same WP twice. 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.
> 
> Signed-off-by: Bhaumik Bhatt <bbhatt@...eaurora.org>
> Signed-off-by: Qiang Yu <quic_qianyu@...cinc.com>
> ---
>   drivers/bus/mhi/host/main.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/bus/mhi/host/main.c b/drivers/bus/mhi/host/main.c
> index dcf627b..13c4b89 100644
> --- a/drivers/bus/mhi/host/main.c
> +++ b/drivers/bus/mhi/host/main.c
> @@ -642,6 +642,7 @@ 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);

This doesn't work due to the write_lock_irqsave(&mhi_chan->lock, flags); 
on line 591.

I really don't like that we are unlocking the mhi_chan while still using 
it.  It opens up a window where the mhi_chan state can be updated 
between here and the client using the callback to queue a buf.

Perhaps we need a new lock that just protects the wp, and needs to be 
only grabbed while mhi_chan->lock is held?

>   			/* notify client */
>   			mhi_chan->xfer_cb(mhi_chan->mhi_dev, &result);
>   
> @@ -667,6 +668,7 @@ static int parse_xfer_event(struct mhi_controller *mhi_cntrl,
>   					kfree(buf_info->cb_buf);
>   				}
>   			}
> +			read_lock_bh(&mhi_chan->lock);
>   		}
>   		break;
>   	} /* CC_EOT */
> @@ -1204,6 +1206,9 @@ int mhi_gen_tre(struct mhi_controller *mhi_cntrl, struct mhi_chan *mhi_chan,
>   	int eot, eob, chain, bei;
>   	int ret;
>   
> +	/* Protect accesses for reading and incrementing WP */
> +	write_lock_bh(&mhi_chan->lock);
> +
>   	buf_ring = &mhi_chan->buf_ring;
>   	tre_ring = &mhi_chan->tre_ring;
>   
> @@ -1221,8 +1226,10 @@ int mhi_gen_tre(struct mhi_controller *mhi_cntrl, struct mhi_chan *mhi_chan,
>   
>   	if (!info->pre_mapped) {
>   		ret = mhi_cntrl->map_single(mhi_cntrl, buf_info);
> -		if (ret)
> +		if (ret) {
> +			write_unlock_bh(&mhi_chan->lock);
>   			return ret;
> +		}
>   	}
>   
>   	eob = !!(flags & MHI_EOB);
> @@ -1239,6 +1246,8 @@ int mhi_gen_tre(struct mhi_controller *mhi_cntrl, struct mhi_chan *mhi_chan,
>   	mhi_add_ring_element(mhi_cntrl, tre_ring);
>   	mhi_add_ring_element(mhi_cntrl, buf_ring);
>   
> +	write_unlock_bh(&mhi_chan->lock);
> +
>   	return 0;
>   }
>