Subject: [PATCH v2] ARM: Don't use complete() during __cpu_die

From: [email-masked] (Stephen Boyd)
Date: Mon, 09 Feb 2015 17:24:08 -0800

On 02/05/15 08:11, Russell King - ARM Linux wrote:
> On Thu, Feb 05, 2015 at 06:29:18AM -0800, Paul E. McKenney wrote:
>> Works for me, assuming no hidden uses of RCU in the IPI code.  ;-)
> Sigh... I kind'a new it wouldn't be this simple.  The gic code which
> actually raises the IPI takes a raw spinlock, so it's not going to be
> this simple - there's a small theoretical window where we have taken
> this lock, written the register to send the IPI, and then dropped the
> lock - the update to the lock to release it could get lost if the
> CPU power is quickly cut at that point.

Hm.. at first glance it would seem like a similar problem exists with
the completion variable. But it seems that we rely on the call to
complete() fom the dying CPU to synchronize with wait_for_completion()
on the killing CPU via the completion's wait.lock.

void complete(struct completion *x)
{
        unsigned long flags;

        spin_lock_irqsave(&x->wait.lock, flags);
        x->done++;
        __wake_up_locked(&x->wait, TASK_NORMAL, 1);
        spin_unlock_irqrestore(&x->wait.lock, flags);
}

and

static inline long __sched
do_wait_for_common(struct completion *x,
                  long (*action)(long), long timeout, int state)
                        ...
			spin_unlock_irq(&x->wait.lock);
			timeout = action(timeout);
			spin_lock_irq(&x->wait.lock);

so the power can't really be cut until the killing CPU sees the lock
released either explicitly via the second cache flush in cpu_die() or
implicitly via hardware. Maybe we can do the same thing here by using a
spinlock for synchronization between the IPI handler and the dying CPU?
So lock/unlock around the IPI sending from the dying CPU and then do a
lock/unlock on the killing CPU before continuing.

It would be nice if we didn't have to do anything at all though so
perhaps we can make it a nop on configs where there isn't a big little
switcher. Yeah it's some ugly coupling between these two pieces of code,
but I'm not sure how we can do better.

>
> Also, we _do_ need the second cache flush in place to ensure that the
> unlock is seen to other CPUs.
>
> We could work around that by taking and releasing the lock in the IPI
> processing function... but this is starting to look less attractive
> as the lock is private to irq-gic.c.

With Daniel Thompson's NMI fiq patches at least the lock would almost
always be gone, except for the bL switcher users. Another solution might
be to put a hotplug lock around the bL switcher code and then skip
taking the lock in gic_raise_softirq() if the IPI is our special hotplug
one. Conditional locking is pretty ugly though, so perhaps this isn't
such a great idea.

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