netdev - Re: [PATCH 5/7] ibmvnic: use a lock to serialize remove/reset

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Date:   Sun, 10 Jan 2021 19:52:25 -0800
From:   Sukadev Bhattiprolu <sukadev@...ux.ibm.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, Dany Madden <drt@...ux.ibm.com>,
        Lijun Pan <ljp@...ux.ibm.com>
Subject: Re: [PATCH 5/7] ibmvnic: use a lock to serialize remove/reset

Jakub Kicinski [kuba@...nel.org] wrote:
> On Thu,  7 Jan 2021 23:12:34 -0800 Sukadev Bhattiprolu wrote:
> > Use a separate lock to serialze ibmvnic_reset() and ibmvnic_remove()
> > functions. ibmvnic_reset() schedules work for the worker thread and
> > ibmvnic_remove() flushes the work before removing the adapter. We
> > don't want any work to be scheduled once we start removing the
> > adapter (i.e after we have already flushed the work).
> 
> Locking based on functions, not on data being accessed is questionable
> IMO. If you don't want work to be scheduled isn't it enough to have a
> bit / flag that you set to let other flows know not to schedule reset?

Maybe I could improve the description, but the "data" being protected
is the work queue. Basically don't add to the work queue while/after
it is (being) flushed.

Existing code is checking for the VNIC_REMOVING state before scheduling
the work but without a lock. If state goes to REMOVING after we check,
we could schedule work after the flush?
> 
> > @@ -5459,6 +5464,7 @@ static int ibmvnic_remove(struct vio_dev *dev)
> >  {
> >  	struct net_device *netdev = dev_get_drvdata(&dev->dev);
> >  	struct ibmvnic_adapter *adapter = netdev_priv(netdev);
> > +	unsigned long rmflags;
> >  	unsigned long flags;
> >  
> >  	spin_lock_irqsave(&adapter->state_lock, flags);
> > @@ -5467,7 +5473,15 @@ static int ibmvnic_remove(struct vio_dev *dev)
> >  		return -EBUSY;
> >  	}
> 
> > +	spin_lock_irqsave(&adapter->remove_lock, rmflags);
> 
> You can just use flags again, no need for separate variables.

Ok.
> 
> >  	adapter->state = VNIC_REMOVING;
> > +	spin_unlock_irqrestore(&adapter->remove_lock, rmflags);