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

[Jeremy Fitzhardinge <jeremy@goop.org>]

Ingo Molnar wrote:
> * 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.

Are you sure?  I thought this is exactly the case where
spin_lock_nest_lock() is supposed to be used?  Admittedly it's very
simple, but the extra lock does two things: 1) it guarantees that the
queue locks can be taken in any order, and 2) it's a single lock on
which we can do spin_lock_irq(), rather than doing it in the loop for
each individual lock (which I think was bogus).

I don't see how it weakens lockdep in any way.  Does it hide any
potential lock misuse?

    J