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

[Qiang Yu <quic_qianyu@quicinc.com>]

On 9/29/2023 11:22 PM, Jeffrey Hugo wrote:
> 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.

I don't think read_lock_bh(&mhi_chan->lock) can protect runtime_pm and 
the packet count here. Even if we do not unlock, mhi state and packet 
count can still be changed because we did not get pm_lock here, which is 
used in all mhi state transition function.

I also checked all places that mhi_chan->lock is grabbed, did not see 
packet count and runtime_pm be protected by write_lock(&mhi_chan->lock).


If you really don't like the unlock operation, we can also take a new 
lock. But I think we only need to add the new lock in two places, 
mhi_gen_tre and mhi_pm_m0_transition 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;
>>>>   }
>>>
>