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stop_machine: Remove stop_cpus_lock and lg_double_lock/unlock() 

stop_two_cpus() and stop_cpus() use stop_cpus_lock to avoid the deadlock,
we need to ensure that the stopper functions can't be queued "backwards"
from one another. This doesn't look nice; if we use lglock then we do not
really need stopper->lock, cpu_stop_queue_work() could use lg_local_lock()
under local_irq_save().

OTOH it would be even better to avoid lglock in stop_machine.c and remove
lg_double_lock(). This patch adds "bool stop_cpus_in_progress" set/cleared
by queue_stop_cpus_work(), and changes cpu_stop_queue_two_works() to busy
wait until it is cleared.

queue_stop_cpus_work() sets stop_cpus_in_progress = T lockless, but after
it queues a work on CPU1 it must be visible to stop_two_cpus(CPU1, CPU2)
which checks it under the same lock. And since stop_two_cpus() holds the
2nd lock too, queue_stop_cpus_work() can not clear stop_cpus_in_progress
if it is also going to queue a work on CPU2, it needs to take that 2nd
lock to do this.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20151121181148.GA433@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Oleg Nesterov 2015-11-21 19:11:48 +01:00 committed by Ingo Molnar
parent 87709e28dc
commit e625397041
3 changed files with 26 additions and 43 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
include/linux/lglock.h
View File

@ -52,15 +52,10 @@ 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);

22
kernel/locking/lglock.c
View File

@ -60,28 +60,6 @@ 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;

42
kernel/stop_machine.c
View File

@ -20,7 +20,6 @@
#include <linux/kallsyms.h>
#include <linux/smpboot.h>
#include <linux/atomic.h>
#include <linux/lglock.h>
#include <linux/nmi.h>
/*
@ -47,13 +46,9 @@ struct cpu_stopper {
static DEFINE_PER_CPU(struct cpu_stopper, cpu_stopper);
static bool stop_machine_initialized = false;
/*
* Avoids a race between stop_two_cpus and global stop_cpus, where
* the stoppers could get queued up in reverse order, leading to
* system deadlock. Using an lglock means stop_two_cpus remains
* relatively cheap.
*/
DEFINE_STATIC_LGLOCK(stop_cpus_lock);
/* static data for stop_cpus */
static DEFINE_MUTEX(stop_cpus_mutex);
static bool stop_cpus_in_progress;
static void cpu_stop_init_done(struct cpu_stop_done *done, unsigned int nr_todo)
{
@ -230,14 +225,26 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1,
struct cpu_stopper *stopper1 = per_cpu_ptr(&cpu_stopper, cpu1);
struct cpu_stopper *stopper2 = per_cpu_ptr(&cpu_stopper, cpu2);
int err;
lg_double_lock(&stop_cpus_lock, cpu1, cpu2);
retry:
spin_lock_irq(&stopper1->lock);
spin_lock_nested(&stopper2->lock, SINGLE_DEPTH_NESTING);
err = -ENOENT;
if (!stopper1->enabled || !stopper2->enabled)
goto unlock;
/*
* Ensure that if we race with __stop_cpus() the stoppers won't get
* queued up in reverse order leading to system deadlock.
*
* We can't miss stop_cpus_in_progress if queue_stop_cpus_work() has
* queued a work on cpu1 but not on cpu2, we hold both locks.
*
* It can be falsely true but it is safe to spin until it is cleared,
* queue_stop_cpus_work() does everything under preempt_disable().
*/
err = -EDEADLK;
if (unlikely(stop_cpus_in_progress))
goto unlock;
err = 0;
__cpu_stop_queue_work(stopper1, work1);
@ -245,8 +252,12 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1,
unlock:
spin_unlock(&stopper2->lock);
spin_unlock_irq(&stopper1->lock);
lg_double_unlock(&stop_cpus_lock, cpu1, cpu2);
if (unlikely(err == -EDEADLK)) {
while (stop_cpus_in_progress)
cpu_relax();
goto retry;
}
return err;
}
/**
@ -316,9 +327,6 @@ bool stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
return cpu_stop_queue_work(cpu, work_buf);
}
/* static data for stop_cpus */
static DEFINE_MUTEX(stop_cpus_mutex);
static bool queue_stop_cpus_work(const struct cpumask *cpumask,
cpu_stop_fn_t fn, void *arg,
struct cpu_stop_done *done)
@ -332,7 +340,8 @@ static bool queue_stop_cpus_work(const struct cpumask *cpumask,
* preempted by a stopper which might wait for other stoppers
* to enter @fn which can lead to deadlock.
*/
lg_global_lock(&stop_cpus_lock);
preempt_disable();
stop_cpus_in_progress = true;
for_each_cpu(cpu, cpumask) {
work = &per_cpu(cpu_stopper.stop_work, cpu);
work->fn = fn;
@ -341,7 +350,8 @@ static bool queue_stop_cpus_work(const struct cpumask *cpumask,
if (cpu_stop_queue_work(cpu, work))
queued = true;
}
lg_global_unlock(&stop_cpus_lock);
stop_cpus_in_progress = false;
preempt_enable();
return queued;
}