Re: [Xen-devel] [PATCHv1 3/4] spinlock: shrink struct lock_debug

[Jennifer Herbert <Jennifer.Herbert@xxxxxxxxxx>]

On 18/12/15 16:58, Jan Beulich wrote:
> > > > On 18.12.15 at 15:09, <david.vrabel@xxxxxxxxxx> wrote:
> > --- a/xen/common/spinlock.c
> > +++ b/xen/common/spinlock.c
> > @@ -16,7 +16,7 @@ static atomic_t spin_debug __read_mostly = ATOMIC_INIT(0);
> >   
> >   static void check_lock(struct lock_debug *debug)
> >   {
> > -    int irq_safe = !local_irq_is_enabled();
> > +    s16 irq_safe = !local_irq_is_enabled();
> >   
> >       if ( unlikely(atomic_read(&spin_debug) <= 0) )
> >           return;
> I can't figure out why this odd looking change is needed.
>
> Jan

This patch shrinks struct lock_debug's irq_safe member.  Since you end 
up with many instances of this embedded in other structures such as 
struct domain, this become a useful saving in memory.  At one point, I 
had an issue with struct domain exceeding 4k, which caused trouble.
Given the change to struct_lock debug,  this local variable irq_safe, is 
compared with it, and used in cmpxchg with it it, and therefore should 
match type.
- Jenny

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