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

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Date:   Wed, 29 Nov 2023 11:29:07 +0800
From:   Qiang Yu <quic_qianyu@...cinc.com>
To:     Manivannan Sadhasivam <mani@...nel.org>
CC:     <quic_jhugo@...cinc.com>, <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 v4 2/4] bus: mhi: host: Drop chan lock before queuing
 buffers


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?
>
> 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@...cinc.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
>>>>
>>>>