On 11/15, Oleg Nesterov wrote:
>
> I am also going to rediff/resend my old patch which removes lglock
> from stop_machine.c, but it probably needs more discussion so I'll
> send it separately.
Please see V2. It is much simpler, and it doesn't need cond_resched().
To me this looks better than changing stop_cpus() to take all stopper
locks at once. Because this actually turns stopper->lock into another
lglock.
Yes, with this patch cpu_stop_queue_two_works() spins in busy-wait loop
if it races with stop_cpus(). But lg_double_lock() spins too, and
performance-wise I think this change is a win.
To simplify the review, let me show the code with this patch applied. The
patch simply adds "bool stop_cpus_in_progress" set by queue_stop_cpus_work()
and checked by cpu_stop_queue_two_works().
static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work
*work1,
int cpu2, struct cpu_stop_work
*work2)
{
struct cpu_stopper *stopper1 = per_cpu_ptr(&cpu_stopper, cpu1);
struct cpu_stopper *stopper2 = per_cpu_ptr(&cpu_stopper, cpu2);
int err;
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);
__cpu_stop_queue_work(stopper2, work2);
unlock:
spin_unlock(&stopper2->lock);
spin_unlock_irq(&stopper1->lock);
if (unlikely(err == -EDEADLK)) {
while (stop_cpus_in_progress)
cpu_relax();
goto retry;
}
return err;
}
static bool queue_stop_cpus_work(const struct cpumask *cpumask,
cpu_stop_fn_t fn, void *arg,
struct cpu_stop_done *done)
{
struct cpu_stop_work *work;
unsigned int cpu;
bool queued = false;
/*
* Disable preemption while queueing to avoid getting
* preempted by a stopper which might wait for other stoppers
* to enter @fn which can lead to deadlock.
*/
preempt_disable();
stop_cpus_in_progress = true;
for_each_cpu(cpu, cpumask) {
work = &per_cpu(cpu_stopper.stop_work, cpu);
work->fn = fn;
work->arg = arg;
work->done = done;
if (cpu_stop_queue_work(cpu, work))
queued = true;
}
stop_cpus_in_progress = false;
preempt_enable();
return queued;
}
Oleg.
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