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Date:   Fri, 20 Oct 2023 09:07:35 -0600
From:   Jeffrey Hugo <quic_jhugo@...cinc.com>
To:     Qiang Yu <quic_qianyu@...cinc.com>, <mani@...nel.org>
CC:     <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 v2 1/2] bus: mhi: host: Add spinlock to protect WP access
 when queueing TREs

On 10/16/2023 2:46 AM, Qiang Yu wrote:
> 
> 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@...eaurora.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@...eaurora.org>
>>>>> Signed-off-by: Qiang Yu <quic_qianyu@...cinc.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.

Mani, if I recall correctly, you were the architect of the locking.  Do 
you have an opinion?

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