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Message-ID: <dfeb6071-8ae4-38ba-5273-59478ea8e178@quicinc.com>
Date: Mon, 25 Sep 2023 11:10:06 +0800
From: Qiang Yu <quic_qianyu@...cinc.com>
To: Jeffrey Hugo <quic_jhugo@...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 9/22/2023 10:44 PM, Jeffrey Hugo wrote:
> 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.
Write_lock_irqsave(&mhi_chan->lock, flags) is used in case of ev_code >=
MHI_EV_CC_OOB. We only read_lock/read_unlock the mhi_chan while ev_code
< MHI_EV_CC_OOB.
>
> 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?
Since we have employed mhi_chan lock to protect the channel and what we
are concerned here is that client may queue buf to a disabled or stopped
channel, can we check channel state after getting mhi_chan->lock like
line 595.
We can add the check after getting write lock in mhi_gen_tre() and after
getting read lock again here.
>
>> /* 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;
>> }
>
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