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Date:   Wed, 6 Dec 2023 19:18:48 +0530
From:   Manivannan Sadhasivam <mani@kernel.org>
To:     Qiang Yu <quic_qianyu@quicinc.com>
Cc:     quic_jhugo@quicinc.com, mhi@lists.linux.dev,
        linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org,
        quic_cang@quicinc.com, quic_mrana@quicinc.com
Subject: Re: [PATCH v4 2/4] bus: mhi: host: Drop chan lock before queuing
 buffers
Message-ID: <20231206134848.GG12802@thinkpad>
References: <1699939661-7385-1-git-send-email-quic_qianyu@quicinc.com>
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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

> > 
> > > > 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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