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

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Message-ID: <20210322165439.GR1719932@casper.infradead.org>
Date:   Mon, 22 Mar 2021 16:54:39 +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 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]);
 
        /* 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;
 
        if (!dev_maps || tc >= dev_maps->num_tc)
                goto out_no_maps;

(or do we need the rcu read lock to read dev->num_rcx_queues? i'm assuming
we only need it to read the xps_maps array)