Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 46c498c2 authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

stop_machine: Mark per cpu stopper enabled early 



commit 14e568e7 (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: default avatarKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reported-and-tested-by: default avatarSander 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: default avatarThomas Gleixner <tglx@linutronix.de>
parent 1a13c0b1

include/linux/smpboot.h

+4 −0
Original line number Diff line number Diff line
@@ -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;

};

kernel/smpboot.c

+2 −0
Original line number Diff line number Diff line
@@ -209,6 +209,8 @@ static void smpboot_unpark_thread(struct smp_hotplug_thread *ht, unsigned int cp 
{

	struct task_struct *tsk = *per_cpu_ptr(ht->store, cpu);



	if (ht->pre_unpark)

		ht->pre_unpark(cpu);

	kthread_unpark(tsk);

}

kernel/stop_machine.c

+1 −1
Original line number Diff line number Diff line
@@ -336,7 +336,7 @@ static struct smp_hotplug_thread cpu_stop_threads = { 
	.create			= cpu_stop_create,

	.setup			= cpu_stop_unpark,

	.park			= cpu_stop_park,

	.unpark			= cpu_stop_unpark,

	.pre_unpark		= cpu_stop_unpark,

	.selfparking		= true,

};