Re: [BUG] bus: mhi: possible ABBA deadlock in mhi_pm_m0_transition() and mhi_send_cmd()

[Jia-Ju Bai]

On 2021/7/16 14:45, Manivannan Sadhasivam wrote:
> On Thu, Jul 15, 2021 at 09:57:22AM -0700, Bhaumik Bhatt wrote:
> > On 2021-07-15 02:45 AM, Jia-Ju Bai wrote:
> > > Hello,
> > >
> > > I find there is a possible ABBA deadlock in the MHI driver in Linux
> > > 5.10:
> > >
> > > In mhi_pm_m0_transition():
> > > 262:     read_lock_bh(&mhi_cntrl->pm_lock);
> > > 281:     spin_lock_irq(&mhi_cmd->lock);
> > >
> > > In mhi_send_cmd():
> > > 1181:   spin_lock_bh(&mhi_cmd->lock);
> > > 1207:   read_lock_bh(&mhi_cntrl->pm_lock);
> > >
> > > When mhi_pm_m0_transition() and mhi_send_cmd() are concurrently
> > > executed, the deadlock can occur.
> > >
> > > I check the code and find a possible case of such concurrent execution:
> > >
> > > #CPU1:
> > > mhi_poll (mhi_event->process_event(...))
> > >    mhi_process_ctrl_ev_ring
> > >      mhi_pm_m0_transition
> > >
> > > #CPU2:
> > > mhi_prepare_for_transfer
> > >    mhi_prepare_channel
> > >      mhi_send_cmd
> > >
> > > Note that mhi_poll() and mhi_prepare_for_transfer() are both exported
> > > by EXPORT_SYMBOL.
> > > Thus, I guess these two functions could be concurrently called by a MHI
> > > driver.
> > >
> > > I am not quite sure whether this possible deadlock is real and how to
> > > fix it if it is real.
> > > Any feedback would be appreciated, thanks :)
> > >
> > >
> > > Best wishes,
> > > Jia-Ju Bai
> > Few pointers from your example:
> >
> > 1. mhi_poll() is currently not used by any client upstream yet.
> Then this shouldn't be added in first place... :/
>
> > 2. Polling is not to be used for single event ring (shared control + data)
> > cases
> > since it is meant to be for client drivers with dedicated data packets only.
> > 3. mhi_send_cmd() will always be called after an mhi_pm_m0_transition() has
> > completed by design since we wait for the device to be held in M0 prior to
> > it.
> But client can be unloaded during M0 event!
>
> Anyway, I don't think the deadlock scenario is valid because of the usage
> of "read_lock_bh()". So if "mhi_send_cmd()" has acquired
> "spin_lock_bh(&mhi_cmd->lock)", it can always acquire
> "read_lock_bh(&mhi_cntrl->pm_lock)" as multiple readers can acquire the
> read lock.
>
> Deadlock would only occur if one of the functions take write lock.
>
> Thanks for auditing.

Thanks for the feedback and explanation :)


Best wishes,
Jia-Ju Bai