Messages in this thread | | | Date | Mon, 8 Dec 2014 11:08:04 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v1] CPU hotplug: active_reader not woken up in some cases - deadlock |
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On Mon, Dec 08, 2014 at 07:58:14PM +0100, David Hildenbrand wrote: > > 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?
You lost me on that one. Don't we get to that piece of code precisely because we don't hold any of the CPU-hotplug locks? If so, the writer might well hold all the locks it needs, and might well change cpu_hotplug.active_writer out from under us.
What am I missing here?
Thanx, Paul
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