[ovs-dev] [PATCH v6 06/12] dpif-netdev: Wait an RCU grace period before freeing ports.

[Ben Pfaff]

(I'm taking a look at this patch specifically because Daniele asked me;
I'm not planning to review the whole series.)

On Mon, Mar 28, 2016 at 12:41:40PM -0700, Daniele Di Proietto wrote:
> The dpif-netdev datapath keeps ports in a cmap which is written only by
> the main thread (holding port_mutex), but which is read concurrently by
> many threads (most notably the pmd threads).
> 
> When removing ports from the datapath we should postpone the deletion,
> otherwise another thread might access invalid memory while reading the
> cmap.
> 
> This commit splits do_port_del() in do_port_remove() and
> do_port_destroy(): the former removes the port from the cmap, while the
> latter reclaims the memory and drops the reference to the underlying
> netdev.

s/del_port/port_del/ here:

> dpif_netdev_del_port() now uses ovsrcu_synchronize() before calling
> do_port_destroy(), to avoid memory corruption in concurrent readers.

ovsrcu_synchronize() requires that nothing in the thread that calls it
is relying on RCU to keep objects around.  That means that no caller of
dfpi_port_del()--there are a few of them--can rely on it.  This is
usually a risky assumption, especially because this assumption can
change later.  Is there reason to believe that it isn't important in all
of these cases?