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
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