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

[Qiang Yu]

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@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