Re: [PATCH v2] CPU hotplug: active_writer not woken up in some cases - deadlock

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

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 <dahi@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
>