stop_machine: Mark per cpu stopper enabled early

commit 14e568e78 (stop_machine: Use smpboot threads) introduced the
following regression:

Before this commit the stopper enabled bit was set in the online
notifier.

CPU0				CPU1
cpu_up
				cpu online
hotplug_notifier(ONLINE)
  stopper(CPU1)->enabled = true;
...
stop_machine()

The conversion to smpboot threads moved the enablement to the wakeup
path of the parked thread. The majority of users seem to have the
following working order:

CPU0				CPU1
cpu_up
				cpu online
unpark_threads()
  wakeup(stopper[CPU1])
....
				stopper thread runs
				  stopper(CPU1)->enabled = true;
stop_machine()

But Konrad and Sander have observed:

CPU0				CPU1
cpu_up
				cpu online
unpark_threads()
  wakeup(stopper[CPU1])
....
stop_machine()
				stopper thread runs
				  stopper(CPU1)->enabled = true;

Now the stop machinery kicks CPU0 into the stop loop, where it gets
stuck forever because the queue code saw stopper(CPU1)->enabled ==
false, so CPU0 waits for CPU1 to enter stomp_machine, but the CPU1
stopper work got discarded due to enabled == false.

Add a pre_unpark function to the smpboot thread descriptor and call it
before waking the thread.

This fixes the problem at hand, but the stop_machine code should be
more robust. The stopper->enabled flag smells fishy at best.

Thanks to Konrad for going through a loop of debug patches and
providing the information to decode this issue.

Reported-and-tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reported-and-tested-by: Sander Eikelenboom <linux@eikelenboom.it>
Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Link: http://lkml.kernel.org/r/alpine.LFD.2.02.1302261843240.22263@ionos
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/smpboot.h b/include/linux/smpboot.h
index c65dee0..13e9296 100644
--- a/include/linux/smpboot.h
+++ b/include/linux/smpboot.h
@@ -24,6 +24,9 @@ struct smpboot_thread_data;
  *			parked (cpu offline)
  * @unpark:		Optional unpark function, called when the thread is
  *			unparked (cpu online)
+ * @pre_unpark:		Optional unpark function, called before the thread is
+ *			unparked (cpu online). This is not guaranteed to be
+ *			called on the target cpu of the thread. Careful!
  * @selfparking:	Thread is not parked by the park function.
  * @thread_comm:	The base name of the thread
  */
@@ -37,6 +40,7 @@ struct smp_hotplug_thread {
 	void				(*cleanup)(unsigned int cpu, bool online);
 	void				(*park)(unsigned int cpu);
 	void				(*unpark)(unsigned int cpu);
+	void				(*pre_unpark)(unsigned int cpu);
 	bool				selfparking;
 	const char			*thread_comm;
 };