Hi Sebastian,
On 19/08/2019 08:33, Sebastian Andrzej Siewior wrote:
On 2019-08-16 17:32:38 [+0100], Julien Grall wrote:
Hi Sebastian,
Hi Julien,
hrtimer_callback_running() will be returning true as the callback is
running somewhere else. This means hrtimer_try_to_cancel()
would return -1. Therefore hrtimer_grab_expiry_lock() would
be called.
Did I miss anything?
nope, you are right. I assumed that we had code to deal with this but
didn't find it…
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 7d7db88021311..40d83c709503e 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -934,7 +934,7 @@ void hrtimer_grab_expiry_lock(const struct hrtimer *timer)
{
struct hrtimer_clock_base *base = timer->base;
- if (base && base->cpu_base) {
+ if (base && base->cpu_base && base->index < MASK_SHIFT) {
Lower indexes are used for hard interrupt. So this would need to be base->index
>= MASK_SHIFT.
But I was wondering whether checking timer->is_soft would make the code more
readable?
While investigation how this is meant to work, I noticed a few others things.
timer->base could potentially change under our feet at any point of time (we
don't hold any lock). So it would be valid to have base == migration_base.
migration_cpu_base does not have softirq_expiry_lock initialized. So we would
end up to use an uninitialized lock. Note that migration_base->index is always
0, so the check base->index > MASK_SHIFT would hide it.
Alternatively, we could initialize the spin lock for migration_cpu_base avoiding
to rely on side effect of the check.
Another potential issue is the compiler is free to reload timer->base at any
time. So I think we want an ACCESS_ONCE(...).
Lastly timer->base cannot be NULL. From the comment on top of
migration_cpu_base, timer->base->cpu_base will as well not be NULL.
So I think the function can be reworked as:
void hrtimer_grab_expirty_lock(const struct hrtimer *timer)
{
struct hrtimer_clock_base *base = ACCESS_ONCE(timer->base);
if (!timer->is_soft && base != migration_base ) {
spin_lock();
spin_unlock();
}
}
spin_lock(&base->cpu_base->softirq_expiry_lock);
spin_unlock(&base->cpu_base->softirq_expiry_lock);
}
This should deal with it.
Cheers,
Sebastian
--
Julien Grall
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