Commit b17718d0 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar

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

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 <>
Signed-off-by: default avatarPeter Zijlstra (Intel) <>
Cc: Andrew Morton <>
Cc: Borislav Petkov <>
Cc: H. Peter Anvin <>
Cc: Linus Torvalds <>
Cc: Oleg Nesterov <>
Cc: Peter Zijlstra <>
Cc: Rik van Riel <>
Cc: Thomas Gleixner <>
Link: default avatarIngo Molnar <>
parent 82a0d276
......@@ -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);
......@@ -60,6 +60,28 @@ void lg_local_unlock_cpu(struct lglock *lg, int 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);
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));
void lg_global_lock(struct lglock *lg)
int i;
......@@ -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;
......@@ -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;
* 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),
&call_args, 1);
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);
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