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

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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;
>>   }
>