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Message-ID: <15526b95-518c-445a-be64-6a15259405fb@quicinc.com>
Date: Mon, 16 Oct 2023 16:46:41 +0800
From: Qiang Yu <quic_qianyu@...cinc.com>
To: Jeffrey Hugo <quic_jhugo@...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 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.
>
>>
>>>
>>>> /* notify client */
>>>> mhi_chan->xfer_cb(mhi_chan->mhi_dev, &result);
>>>> @@ -667,6 +668,7 @@ static int parse_xfer_event(struct
>>>> mhi_controller *mhi_cntrl,
>>>> kfree(buf_info->cb_buf);
>>>> }
>>>> }
>>>> + read_lock_bh(&mhi_chan->lock);
>>>> }
>>>> break;
>>>> } /* CC_EOT */
>>>> @@ -1204,6 +1206,9 @@ int mhi_gen_tre(struct mhi_controller
>>>> *mhi_cntrl, struct mhi_chan *mhi_chan,
>>>> int eot, eob, chain, bei;
>>>> int ret;
>>>> + /* Protect accesses for reading and incrementing WP */
>>>> + write_lock_bh(&mhi_chan->lock);
>>>> +
>>>> buf_ring = &mhi_chan->buf_ring;
>>>> tre_ring = &mhi_chan->tre_ring;
>>>> @@ -1221,8 +1226,10 @@ int mhi_gen_tre(struct mhi_controller
>>>> *mhi_cntrl, struct mhi_chan *mhi_chan,
>>>> if (!info->pre_mapped) {
>>>> ret = mhi_cntrl->map_single(mhi_cntrl, buf_info);
>>>> - if (ret)
>>>> + if (ret) {
>>>> + write_unlock_bh(&mhi_chan->lock);
>>>> return ret;
>>>> + }
>>>> }
>>>> eob = !!(flags & MHI_EOB);
>>>> @@ -1239,6 +1246,8 @@ int mhi_gen_tre(struct mhi_controller
>>>> *mhi_cntrl, struct mhi_chan *mhi_chan,
>>>> mhi_add_ring_element(mhi_cntrl, tre_ring);
>>>> mhi_add_ring_element(mhi_cntrl, buf_ring);
>>>> + write_unlock_bh(&mhi_chan->lock);
>>>> +
>>>> return 0;
>>>> }
>>>
>
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