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Commit b17718d0 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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sched/stop_machine: Fix deadlock between multiple stop_two_cpus() 



Jiri reported a machine stuck in multi_cpu_stop() with
migrate_swap_stop() as function and with the following src,dst cpu
pairs: {11,  4} {13, 11} { 4, 13}

                        4       11      13

cpuM: queue(4 ,13)
                        *Ma
cpuN: queue(13,11)
                                *N      Na
                        *M              Mb
cpuO: queue(11, 4)
                        *O      Oa
                                *Nb
                        *Ob

Where *X denotes the cpu running the queueing of cpu-X and X[ab] denotes
the first/second queued work.

You'll observe the top of the workqueue for each cpu: 4,11,13 to be work
from cpus: M, O, N resp. IOW. deadlock.

Do away with the queueing trickery and introduce lg_double_lock() to
lock both CPUs and fully serialize the stop_two_cpus() callers instead
of the partial (and buggy) serialization we have now.

Reported-by: default avatarJiri Olsa <jolsa@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150605153023.GH19282@twins.programming.kicks-ass.net


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 82a0d276

include/linux/lglock.h

+5 −0
Original line number Diff line number Diff line
@@ -52,10 +52,15 @@ struct lglock { 
	static struct lglock name = { .lock = &name ## _lock }



void lg_lock_init(struct lglock *lg, char *name);



void lg_local_lock(struct lglock *lg);

void lg_local_unlock(struct lglock *lg);

void lg_local_lock_cpu(struct lglock *lg, int cpu);

void lg_local_unlock_cpu(struct lglock *lg, int cpu);



void lg_double_lock(struct lglock *lg, int cpu1, int cpu2);

void lg_double_unlock(struct lglock *lg, int cpu1, int cpu2);



void lg_global_lock(struct lglock *lg);

void lg_global_unlock(struct lglock *lg);

kernel/locking/lglock.c

+22 −0
Original line number Diff line number Diff line
@@ -60,6 +60,28 @@ void lg_local_unlock_cpu(struct lglock *lg, int cpu) 
}

EXPORT_SYMBOL(lg_local_unlock_cpu);



void lg_double_lock(struct lglock *lg, int cpu1, int cpu2)

{

	BUG_ON(cpu1 == cpu2);



	/* lock in cpu order, just like lg_global_lock */

	if (cpu2 < cpu1)

		swap(cpu1, cpu2);



	preempt_disable();

	lock_acquire_shared(&lg->lock_dep_map, 0, 0, NULL, _RET_IP_);

	arch_spin_lock(per_cpu_ptr(lg->lock, cpu1));

	arch_spin_lock(per_cpu_ptr(lg->lock, cpu2));

}



void lg_double_unlock(struct lglock *lg, int cpu1, int cpu2)

{

	lock_release(&lg->lock_dep_map, 1, _RET_IP_);

	arch_spin_unlock(per_cpu_ptr(lg->lock, cpu1));

	arch_spin_unlock(per_cpu_ptr(lg->lock, cpu2));

	preempt_enable();

}



void lg_global_lock(struct lglock *lg)

{

	int i;

kernel/stop_machine.c

+5 −37
Original line number Diff line number Diff line
@@ -211,25 +211,6 @@ static int multi_cpu_stop(void *data) 
	return err;

}



struct irq_cpu_stop_queue_work_info {

	int cpu1;

	int cpu2;

	struct cpu_stop_work *work1;

	struct cpu_stop_work *work2;

};



/*

 * This function is always run with irqs and preemption disabled.

 * This guarantees that both work1 and work2 get queued, before

 * our local migrate thread gets the chance to preempt us.

 */

static void irq_cpu_stop_queue_work(void *arg)

{

	struct irq_cpu_stop_queue_work_info *info = arg;

	cpu_stop_queue_work(info->cpu1, info->work1);

	cpu_stop_queue_work(info->cpu2, info->work2);

}



/**

 * stop_two_cpus - stops two cpus

 * @cpu1: the cpu to stop
@@ -245,7 +226,6 @@ int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void * 
{

	struct cpu_stop_done done;

	struct cpu_stop_work work1, work2;

	struct irq_cpu_stop_queue_work_info call_args;

	struct multi_stop_data msdata;



	preempt_disable();
@@ -262,13 +242,6 @@ int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void * 
		.done = &done

	};



	call_args = (struct irq_cpu_stop_queue_work_info){

		.cpu1 = cpu1,

		.cpu2 = cpu2,

		.work1 = &work1,

		.work2 = &work2,

	};



	cpu_stop_init_done(&done, 2);

	set_state(&msdata, MULTI_STOP_PREPARE);
@@ -285,16 +258,11 @@ int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void * 
		return -ENOENT;

	}



	lg_local_lock(&stop_cpus_lock);

	/*

	 * Queuing needs to be done by the lowest numbered CPU, to ensure

	 * that works are always queued in the same order on every CPU.

	 * This prevents deadlocks.

	 */

	smp_call_function_single(min(cpu1, cpu2),

				 &irq_cpu_stop_queue_work,

				 &call_args, 1);

	lg_local_unlock(&stop_cpus_lock);

	lg_double_lock(&stop_cpus_lock, cpu1, cpu2);

	cpu_stop_queue_work(cpu1, &work1);

	cpu_stop_queue_work(cpu2, &work2);

	lg_double_unlock(&stop_cpus_lock, cpu1, cpu2);



	preempt_enable();



	wait_for_completion(&done.completion);