Commit 46c498c2 authored Feb 26, 2013 by Thomas Gleixner

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: 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>

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,
		};

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