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Message-ID: <c3e628bd-20cf-4196-959a-c178e4452773@quicinc.com>
Date:   Thu, 7 Dec 2023 17:50:22 +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>
 <639d6008-bdfa-4b6e-b622-e916003ec908@quicinc.com>
 <20231128133252.GX3088@thinkpad>
 <5eb0a521-0b72-4d15-9a65-429c4c123833@quicinc.com>
 <20231130053157.GB3043@thinkpad>
 <9873ee7b-7ef1-4327-8e22-49e1cd3872f1@quicinc.com>
 <20231206134848.GG12802@thinkpad>
 <947af144-e2a2-49bc-9f39-31f6ee5b1aa1@quicinc.com>
 <20231207064347.GE2932@thinkpad>
From:   Qiang Yu <quic_qianyu@quicinc.com>
In-Reply-To: <20231207064347.GE2932@thinkpad>
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On 12/7/2023 2:43 PM, Manivannan Sadhasivam wrote:
> On Thu, Dec 07, 2023 at 01:27:19PM +0800, Qiang Yu wrote:
>> On 12/6/2023 9:48 PM, Manivannan Sadhasivam wrote:
>>> On Wed, Dec 06, 2023 at 10:25:12AM +0800, Qiang Yu wrote:
>>>> 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.
>>> Not at all. Btw, did you actually encounter any issue that this patch is trying
>>> to fix? Or just fixing based on code inspection.
>>>
>>> - Mani
>> Yes, we actually meet the race issue in downstream driver. But I can not
>> find more details about the issue.
> Hmm. I think it is OK to accept this patch and ignore the channel disabling
> concern since the event lock is in place to prevent that. There would be no
> deadlock as I mentioned above, since the process that is parsing the xfer event
> is not the one that is going to disable the channel in parallel.
>
> Could you please respin this series dropping patch 3/4 and also addressing the
> issue I mentioned in patch 4/4?
>
> - Mani
Thank you for tirelessly review these patches. Will do this in next version.