netdev - Re: [PATCH net-next] net-sysfs: remove possible sleep from an RCU read-side critical section

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Date:   Mon, 22 Mar 2021 18:45:57 +0100
From:   Antoine Tenart <atenart@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     davem@...emloft.net, kuba@...nel.org, alexander.duyck@...il.com,
        netdev@...r.kernel.org, kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH net-next] net-sysfs: remove possible sleep from an RCU read-side critical section

Quoting Antoine Tenart (2021-03-22 18:41:30)
> Quoting Matthew Wilcox (2021-03-22 17:54:39)
> > On Mon, Mar 22, 2021 at 04:43:29PM +0100, Antoine Tenart wrote:
> > > xps_queue_show is mostly made of an RCU read-side critical section and
> > > calls bitmap_zalloc with GFP_KERNEL in the middle of it. That is not
> > > allowed as this call may sleep and such behaviours aren't allowed in RCU
> > > read-side critical sections. Fix this by using GFP_NOWAIT instead.
> > 
> > This would be another way of fixing the problem that is slightly less
> > complex than my initial proposal, but does allow for using GFP_KERNEL
> > for fewer failures:
> > 
> > @@ -1366,11 +1366,10 @@ static ssize_t xps_queue_show(struct net_device *dev, unsigned int index,
> >  {
> >         struct xps_dev_maps *dev_maps;
> >         unsigned long *mask;
> > -       unsigned int nr_ids;
> > +       unsigned int nr_ids, new_nr_ids;
> >         int j, len;
> >  
> > -       rcu_read_lock();
> > -       dev_maps = rcu_dereference(dev->xps_maps[type]);
> > +       dev_maps = READ_ONCE(dev->xps_maps[type]);
> 
> Couldn't dev_maps be freed between here and the read of dev_maps->nr_ids
> as we're not in an RCU read-side critical section?

* The first read of dev_maps->nr_ids, happening before rcu_read_lock,
  not the one shown below.

> >         /* Default to nr_cpu_ids/dev->num_rx_queues and do not just return 0
> >          * when dev_maps hasn't been allocated yet, to be backward compatible.
> > @@ -1379,10 +1378,18 @@ static ssize_t xps_queue_show(struct net_device *dev, unsigned int index,
> >                  (type == XPS_CPUS ? nr_cpu_ids : dev->num_rx_queues);
> >  
> >         mask = bitmap_zalloc(nr_ids, GFP_KERNEL);
> > -       if (!mask) {
> > -               rcu_read_unlock();
> > +       if (!mask)
> >                 return -ENOMEM;
> > -       }
> > +
> > +       rcu_read_lock();
> > +       dev_maps = rcu_dereference(dev->xps_maps[type]);
> > +       /* if nr_ids shrank in the meantime, do not overrun array.
> > +        * if it increased, we just won't show the new ones
> > +        */
> > +       new_nr_ids = dev_maps ? dev_maps->nr_ids :
> > +                       (type == XPS_CPUS ? nr_cpu_ids : dev->num_rx_queues);
> > +       if (new_nr_ids < nr_ids)
> > +               nr_ids = new_nr_ids;