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    SubjectRe: [PATCH v4 2/4] bus: mhi: host: Drop chan lock before queuing buffers
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    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.
    >
    >>> Also, I'm planning to cleanup the locking mess within MHI in the coming days.
    >>> Perhaps we can revisit this series at that point of time. Will that be OK for
    >>> you?
    >> Sure, that will be great.
    >>> - Mani
    >>>
    >>>>>> 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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