Messages in this thread | | | Date | Thu, 7 Dec 2023 13:27:19 +0800 | Subject | Re: [PATCH v4 2/4] bus: mhi: host: Drop chan lock before queuing buffers | From | Qiang Yu <> |
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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. >>>>> 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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