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 17:44:21 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Antoine Tenart <atenart@...nel.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

On Mon, Mar 22, 2021 at 06:41:30PM +0100, Antoine Tenart wrote:
> Quoting Matthew Wilcox (2021-03-22 17:54:39)
> > -       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?

Oh, good point.  Never mind, then.

> My feeling is there is not much value in having a tricky allocation
> logic for reads from xps_cpus and xps_rxqs. While we could come up with
> something, returning -ENOMEM on memory pressure should be fine.

That's fine.  It's your code, and this is probably a small allocation
anyway.