On Mon, Dec 08, 2014 at 09:21:22PM +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. Also move setting of TASK_UNINTERRUPTIBLE in cpu_hotplug_begin() above
> the
> check, so we won't lose any wakeups when racing with put_online_cpus().
>
> Can't reproduce it with this fix.
>
> Signed-off-by: David Hildenbrand <d...@linux.vnet.ibm.com>
> ---
> kernel/cpu.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 90a3d01..1f50c06 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -113,10 +113,16 @@ EXPORT_SYMBOL_GPL(try_get_online_cpus);
>
> void put_online_cpus(void)
> {
> + struct task_struct *active_writer;
> +
> if (cpu_hotplug.active_writer == current)
> return;
> if (!mutex_trylock(&cpu_hotplug.lock)) {
> atomic_inc(&cpu_hotplug.puts_pending);
> + /* we might be the last one */
> + active_writer = cpu_hotplug.active_writer;
The compiler is within its rights to optimize the active_writer local
variable out of existence, thus re-introducing the possible race with
the writer that can pass a NULL pointer to wake_up_process(). So you
really need the ACCESS_ONCE() on the read from cpu_hotplug.active_writer.
Please see http://lwn.net/Articles/508991/ for more information why
this is absolutely required.
> + if (unlikely(active_writer))
> + wake_up_process(active_writer);
> cpuhp_lock_release();
> return;
> }
> @@ -161,15 +167,17 @@ void cpu_hotplug_begin(void)
> cpuhp_lock_acquire();
> for (;;) {
> mutex_lock(&cpu_hotplug.lock);
> + __set_current_state(TASK_UNINTERRUPTIBLE);
You lost me on this one. How does this help?
Thanx, Paul
> if (atomic_read(&cpu_hotplug.puts_pending)) {
> int delta;
>
> delta = atomic_xchg(&cpu_hotplug.puts_pending, 0);
> cpu_hotplug.refcount -= delta;
> }
> - if (likely(!cpu_hotplug.refcount))
> + if (likely(!cpu_hotplug.refcount)) {
> + __set_current_state(TASK_RUNNING);
> break;
> - __set_current_state(TASK_UNINTERRUPTIBLE);
> + }
> mutex_unlock(&cpu_hotplug.lock);
> schedule();
> }
> --
> 1.8.5.5
>
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