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

[Jeffrey Hugo <quic_jhugo@quicinc.com>]

On 9/24/2023 9:10 PM, Qiang Yu wrote:
> 
> On 9/22/2023 10:44 PM, Jeffrey Hugo wrote:
>> On 9/13/2023 2:47 AM, Qiang Yu wrote:
>>> From: Bhaumik Bhatt <bbhatt@codeaurora.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@codeaurora.org>
>>> Signed-off-by: Qiang Yu <quic_qianyu@quicinc.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.

Sorry.  OOB != EOB

>>
>> 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.

I'm not sure that is sufficient.  After you unlock to notify the client, 
MHI is going to manipulate the packet count and runtime_pm without the 
lock (648-652).  It seems like that adds additional races which won't be 
covered by the additional check you propose.

> 
>>
>>>               /* 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;
>>>   }
>>