Re: [PATCH] kernel/timer.c: using spin_lock_irqsave instead of spin_lock + local_irq_save, especially when CONFIG_LOCKDEP not defined

[Chen Gang <gang.chen@asianux.com>]

On 06/20/2013 05:07 PM, Thomas Gleixner wrote:
> On Thu, 20 Jun 2013, Chen Gang wrote:
>> > On 06/19/2013 06:53 PM, Thomas Gleixner wrote:
>>> > > So
>>> > > 	local_irq_save(flags);
>>> > > 	spin_lock(&lock);
>>> > > 
>>> > > is semantically the same as 
>>> > > 
>>> > > 	spin_lock_irqsave(&lock, flags);
>>> > > 
>> > 
>> > Yes (but reverse is NO).
>> > 
>>> > > And this is completely independent of LOCKDEP.
>> > 
>> > NO.
>> > 
>> >  	spin_lock_irqsave(&lock, flags);
>> > 
>> >  is not semantically the same as
>> > 
>> >  	local_irq_save(flags);
>> >  	spin_lock(&lock);
> If A is semantically the same as B, then B is semantically the same as
> A. At least that's the common understanding.
> 

  From A to B is OK.

Not means:

  From B to A is also OK.


> You seem to have a different definition of semantics, but I prefer the
> common one.
>  
>> > It depend on the spin_lock_irqsave() implementation, if the parameters
>> > has no relation ship with each other, semantically the same.
> Yes, it depends on the implementation, but all implementations do:
> 
>      local_irq_save(flags);
>      arch_spin_lock_flags(l, flags);
> 

Yes this is spin_lock_irqsave().

At least, this implemenation is not equal to.

	local_irq_save(flags);
	spin_lock(l);

So if for arch_spin_lock_flags(), 'flags' is no relation ship with 'l',
we can say semantically the same.



Thanks.
-- 
Chen Gang

Asianux Corporation