Subject: Re: [PATCH net 1/6] ice: avoid bonding causing auxiliary plug/unplug under RTNL lock

From: Leon Romanovsky <[email-masked]>
Date: Wed, 15 Feb 2023 14:13:33 +0200

On Tue, Feb 14, 2023 at 10:24:04PM +0000, Ertman, David M wrote:
> > -----Original Message-----
> > From: Leon Romanovsky <[email-masked]>
> > Sent: Wednesday, February 1, 2023 1:50 AM
> > Subject: Re: [PATCH net 1/6] ice: avoid bonding causing auxiliary plug/unplug
> > under RTNL lock
> > 
> > On Tue, Jan 31, 2023 at 01:36:58PM -0800, Tony Nguyen wrote:
> > > From: Dave Ertman <[email-masked]>
> > >
> > > RDMA is not supported in ice on a PF that has been added to a bonded
> > > interface. To enforce this, when an interface enters a bond, we unplug
> > > the auxiliary device that supports RDMA functionality.  This unplug
> > > currently happens in the context of handling the netdev bonding event.
> > > This event is sent to the ice driver under RTNL context.  This is causing
> > > a deadlock where the RDMA driver is waiting for the RTNL lock to complete
> > > the removal.
> > >
> > > Defer the unplugging/re-plugging of the auxiliary device to the service
> > > task so that it is not performed under the RTNL lock context.
> > >
> > > Reported-by: Jaroslav Pulchart <[email-masked]>
> > > Link: https://lore.kernel.org/linux-rdma/68b14b11-d0c7-65c9-4eeb-
> > [email-masked]/
> > > Fixes: 5cb1ebdbc434 ("ice: Fix race condition during interface enslave")
> > > Fixes: 4eace75e0853 ("RDMA/irdma: Report the correct link speed")
> > > Signed-off-by: Dave Ertman <[email-masked]>
> > > Reviewed-by: Michal Swiatkowski <[email-masked]>
> > > Tested-by: Gurucharan G <[email-masked]> (A Contingent
> > worker at Intel)
> > > Signed-off-by: Tony Nguyen <[email-masked]>
> > > ---
> > >  drivers/net/ethernet/intel/ice/ice.h      | 14 +++++---------
> > >  drivers/net/ethernet/intel/ice/ice_main.c | 17 +++++++----------
> > >  2 files changed, 12 insertions(+), 19 deletions(-)
> > 
> > <...>
> > 
> > > index 5f86e4111fa9..055494dbcce0 100644
> > > --- a/drivers/net/ethernet/intel/ice/ice_main.c
> > > +++ b/drivers/net/ethernet/intel/ice/ice_main.c
> > > @@ -2290,18 +2290,15 @@ static void ice_service_task(struct work_struct
> > *work)
> > >  		}
> > >  	}
> > >
> > > -	if (test_bit(ICE_FLAG_PLUG_AUX_DEV, pf->flags)) {
> > > -		/* Plug aux device per request */
> > > +	/* Plug aux device per request */
> > > +	if (test_and_clear_bit(ICE_FLAG_PLUG_AUX_DEV, pf->flags))
> > 
> > Very interesting pattern. You are not holding any locks while running
> > ice_service_task() and clear bits before you actually performed requested
> > operation.
> > 
> > How do you protect from races while testing bits in other places of ice
> > driver?
> 
> Leon,
> 
> Thanks for the review and sorry for the late reply, got sidetracked into another project.
> 
> Your review caused us to re-evaluate the plug/unplug flow, and since these bits are only set/cleared in
> the bonding event flow, and the UNPLUG bit set clears the PLUG bit, we attain the desired outcome
> in all cases if we swap the order that we evaluate the bits in the service task.

I afraid that it won't make ice state machine more understandable. :)

Thanks

> 
> Any multi-event situation that happens between or during service task will be handled in the expected way.
> 
> DaveE
> 
> > 
> > Thanks
> > 
> > >  		ice_plug_aux_dev(pf);
> > >
> > > -		/* Mark plugging as done but check whether unplug was
> > > -		 * requested during ice_plug_aux_dev() call
> > > -		 * (e.g. from ice_clear_rdma_cap()) and if so then
> > > -		 * plug aux device.
> > > -		 */
> > > -		if (!test_and_clear_bit(ICE_FLAG_PLUG_AUX_DEV, pf-
> > >flags))
> > > -			ice_unplug_aux_dev(pf);
> > > -	}
> > > +	/* unplug aux dev per request, if an unplug request came in
> > > +	 * while processing a plug request, this will handle it
> > > +	 */
> > > +	if (test_and_clear_bit(ICE_FLAG_UNPLUG_AUX_DEV, pf->flags))
> > > +		ice_unplug_aux_dev(pf);
> > >
> > >  	if (test_and_clear_bit(ICE_FLAG_MTU_CHANGED, pf->flags)) {
> > >  		struct iidc_event *event;
> > > --
> > > 2.38.1
> > >