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SubjectRe: [PATCH 0 of 9] x86/smp function calls: convert x86 tlb flushes to use function calls [POST 2]

* Jeremy Fitzhardinge <jeremy@goop.org> wrote:

> I think this might be a spurious "holding multiple locks in the same
> class" bug. All the queue locks are presumably in the same class, and
> ipi_call_lock_irq() wants to hold them all to lock out any IPIs.
> Spurious because this is the only place which holds more than one
> queue lock, and it always locks 0->N.
>
> I guess the fix is to use an outer lock and use spin_lock_nested()
> (now that it exists). Something along these lines?

this is not a good idea:

> +/* Hold queues_lock when taking more than one queue[].lock at once */
> +static DEFINE_SPINLOCK(queues_lock);

because it adds an artificial extra spinlock for no good reason and
weakens the lock dependency checking as well.

Just add a lock class descriptor to each call_function_queue lock, so
that lockdep can see that it's truly all in the correct order.

I.e. dont turn lockdep off artificially.

Ingo


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