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

[Qiang Yu]

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?
> 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@xxxxxxxxxxx>
> > > > ---
> > > >    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