> On Mon, Dec 08, 2014 at 07:13:03PM +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 quick 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.
> >
> > Can't reproduce the error with this fix.
>
> Good catch!
>
> But don't we need to use exactly the same value for the NULL check
> and for the wakeup? Otherwise, wouldn't it be possible for
> cpu_hotplug.active_writer to be non-NULL for the check but NULL
> for the wake_up_process()?
>
> Thanx, Paul
active_writer is cleared while holding cpuhp_lock, so this should be safe,
right?
Thanks!
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