Re: [PATCH v3] CPU hotplug: active_writer not woken up in some cases - deadlock

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Dec 09, 2014 at 05:24:27PM +0100, David Hildenbrand wrote:
> > On Tue, Dec 09, 2014 at 01:23:31PM +0100, David Hildenbrand wrote:
> > > Commit b2c4623dcd07 ("rcu: More on deadlock between CPU hotplug and expedited
> > > grace periods") introduced another problem that can easily be reproduced by
> > > starting/stopping cpus in a loop.
> > > 
> > > E.g.:
> > >   for i in `seq 5000`; do
> > >       echo 1 > /sys/devices/system/cpu/cpu1/online
> > >       echo 0 > /sys/devices/system/cpu/cpu1/online
> > >   done
> > > 
> > > Will result in:
> > >   INFO: task /cpu_start_stop:1 blocked for more than 120 seconds.
> > >   Call Trace:
> > >   ([<00000000006a028e>] __schedule+0x406/0x91c)
> > >    [<0000000000130f60>] cpu_hotplug_begin+0xd0/0xd4
> > >    [<0000000000130ff6>] _cpu_up+0x3e/0x1c4
> > >    [<0000000000131232>] cpu_up+0xb6/0xd4
> > >    [<00000000004a5720>] device_online+0x80/0xc0
> > >    [<00000000004a57f0>] online_store+0x90/0xb0
> > >   ...
> > > 
> > > And a deadlock.
> > > 
> > > Problem is that if the last ref in put_online_cpus() can't get the
> > > cpu_hotplug.lock the puts_pending count is incremented, but a sleeping
> > > active_writer might never be woken up, therefore never exiting the loop in
> > > cpu_hotplug_begin().
> > > 
> > > This fix wakes up the active_writer proactively. The writer already goes back to
> > > sleep if the ref count isn't already down to 0, so this should be fine.
> > > 
> > > In order to avoid many potential races, we have to:
> > > - Protect current_writer by a spin lock. When holding this lock we can be sure
> > >   that the writer won't vainsh or change. (use-after-free)
> > > - Increment the cpu_hotplug.puts_pending count before we test for an
> > >   active_writer. (otherwise a wakeup might get lost)
> > > - Move setting of TASK_UNINTERRUPTIBLE in cpu_hotplug_begin() above the
> > >   condition check. (otherwise a wakeup might get lost)
> > > 
> > > Can't reproduce it with this fix.
> > 
> > Would wait_event()/wake_up() work for the wakeup-writer case?
> 
> Thanks! Was also thinking about wait queues. Will investigate if that can help
> to beautify this :)

Looking forward to seeing it.  ;-)

							Thanx, Paul