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SubjectRe: [PATCH v4 2/4] bus: mhi: host: Drop chan lock before queuing buffers
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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
>>
>>

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