Re: [PATCH v4 2/4] bus: mhi: host: Drop chan lock before queuing buffers

[Qiang Yu <quic_qianyu@quicinc.com>]

On 11/30/2023 1:31 PM, Manivannan Sadhasivam wrote:
> On Wed, Nov 29, 2023 at 11:29:07AM +0800, Qiang Yu wrote:
>> On 11/28/2023 9:32 PM, Manivannan Sadhasivam wrote:
>>> On Mon, Nov 27, 2023 at 03:13:55PM +0800, Qiang Yu wrote:
>>>> On 11/24/2023 6:04 PM, Manivannan Sadhasivam wrote:
>>>>> On Tue, Nov 14, 2023 at 01:27:39PM +0800, Qiang Yu wrote:
>>>>>> 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.
>>>>>>
>>>>> Is this patch trying to fix an existing issue in client drivers or a potential
>>>>> issue in the future drivers?
>>>>>
>>>>> Even if you take care of disabled channels, "mhi_event->lock" acquired during
>>>>> mhi_mark_stale_events() can cause deadlock, since event lock is already held by
>>>>> mhi_ev_task().
>>>>>
>>>>> I'd prefer not to open the window unless this patch is fixing a real issue.
>>>>>
>>>>> - Mani
>>>> In [PATCH v4 1/4] bus: mhi: host: Add spinlock to protect WP access when
>>>> queueing
>>>> TREs,  we add
>>>> write_lock_bh(&mhi_chan->lock)/write_unlock_bh(&mhi_chan->lock)
>>>> in mhi_gen_tre, which may be invoked as part of mhi_queue in client xfer
>>>> callback,
>>>> so we have to use read_unlock_bh(&mhi_chan->lock) here to avoid acquiring
>>>> mhi_chan->lock
>>>> twice.
>>>>
>>>> Sorry for confusing you. Do you think we need to sqush this two patch into
>>>> one?
>>> Well, if patch 1 is introducing a potential deadlock, then we should fix patch
>>> 1 itself and not introduce a follow up patch.
>>>
>>> But there is one more issue that I pointed out in my previous reply.
>> Sorry, I can not understand why "mhi_event->lock" acquired during
>> mhi_mark_stale_events() can cause deadlock. In mhi_ev_task(), we will
>> not invoke mhi_mark_stale_events(). Can you provide some interpretation?
> Going by your theory that if a channel gets disabled while processing the event,
> the process trying to disable the channel will try to acquire "mhi_event->lock"
> which is already held by the process processing the event.
>
> - Mani
OK, I get you. Thank you for kind explanation. Hopefully I didn't 
intrude too much.
>
>>> Also, I'm planning to cleanup the locking mess within MHI in the coming days.
>>> Perhaps we can revisit this series at that point of time. Will that be OK for
>>> you?
>> Sure, that will be great.
>>> - Mani
>>>
>>>>>> Signed-off-by: Qiang Yu <quic_qianyu@quicinc.com>
>>>>>> ---
>>>>>>     drivers/bus/mhi/host/main.c | 4 ++++
>>>>>>     1 file changed, 4 insertions(+)
>>>>>>
>>>>>> diff --git a/drivers/bus/mhi/host/main.c b/drivers/bus/mhi/host/main.c
>>>>>> index 6c6d253..c4215b0 100644
>>>>>> --- a/drivers/bus/mhi/host/main.c
>>>>>> +++ b/drivers/bus/mhi/host/main.c
>>>>>> @@ -642,6 +642,8 @@ 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);
>>>>>> +
>>>>>>     			/* notify client */
>>>>>>     			mhi_chan->xfer_cb(mhi_chan->mhi_dev, &result);
>>>>>> @@ -667,6 +669,8 @@ static int parse_xfer_event(struct mhi_controller *mhi_cntrl,
>>>>>>     					kfree(buf_info->cb_buf);
>>>>>>     				}
>>>>>>     			}
>>>>>> +
>>>>>> +			read_lock_bh(&mhi_chan->lock);
>>>>>>     		}
>>>>>>     		break;
>>>>>>     	} /* CC_EOT */
>>>>>> -- 
>>>>>> 2.7.4
>>>>>>
>>>>>>