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SubjectRe: [RT PATCH 3/3] hrtimer: Prevent using uninitialized spin_lock in hrtimer_grab_expiry_lock()
On Wed, 21 Aug 2019, Julien Grall wrote:

> migration_base is used as a placeholder when an hrtimer is switching
> between base (see switch_hrtimer_timer_base). It is possible
> theoritically possible to have timer->base equal to migration_base.
> Even if it is a placeholder, it would pass all the current check in
> hrtimer_grab_expiry_lock() leading to use softirq_expiry_lock
> uninitialized.
> This is can be prevented by checking whether the base is equal to
> the placeholder (i.e. migration_base).

That's a lame argument. The point is that it does not make sense to do that
on migration base, but not for the reason you are giving (uninitialized

If base == migration_base then there is no point to lock soft_expiry_lock
simply because the timer is not executing the callback in soft irq context
and the whole lock/unlock dance can be avoided.

But, yes. Good catch.



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