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Message-ID: <66d37188-f5e0-4f8a-b190-9b3865876cdd@quicinc.com>
Date:   Mon, 27 Nov 2023 15:12:30 +0800
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Subject: Re: [PATCH v4 2/4] bus: mhi: host: Drop chan lock before queuing
 buffers
Content-Language: en-US
To:     Manivannan Sadhasivam <mani@kernel.org>
CC:     <quic_jhugo@quicinc.com>, <mhi@lists.linux.dev>,
        <linux-arm-msm@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <quic_cang@quicinc.com>, <quic_mrana@quicinc.com>
References: <1699939661-7385-1-git-send-email-quic_qianyu@quicinc.com>
 <1699939661-7385-3-git-send-email-quic_qianyu@quicinc.com>
 <20231124100403.GA4536@thinkpad>
From:   Qiang Yu <quic_qianyu@quicinc.com>
In-Reply-To: <20231124100403.GA4536@thinkpad>
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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?
>
>> 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
>>
>>