Linux 4.4-rc4 regression, bisected to "net: fix sock_wake_async() rcu protection"

[Andy Lutomirski]

On recent v4.4-rc releases, I can't run emacs.  No, really, running
"emacs" in a GNOME 3 session makes gnome-shell think that emacs is
running, but no window is drawn, and the overall system UI is a bit
weird when the invisible emacs window is focused.

This is 100% reproducible.

There might be other symptoms involving gdb malfunctioning, but those
are, at best, sporadic.  The emacs failure is entirely reliable.  I
have no idea what the underlying failure mode is, but failure to wake
a socket waiter seems plausible,  I also have no idea why oocalc,
gimp, vim, gedit, firefox, etc aren't affected.

A somewhat unorthodox "git bisect" run blames:

commit ceb5d58b217098a657f3850b7a2640f995032e62
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date:   Sun Nov 29 20:03:11 2015 -0800

    net: fix sock_wake_async() rcu protection

I've confirmed that v4.4-rc7 with that patch reverted works fine.

Since the offending commit was apparently a security fix, simply
reverting it might not be the best idea.

--Andy
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