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

[Jennifer Herbert <Jennifer.Herbert@citrix.com>]

On 18/12/15 16:58, Jan Beulich wrote:
>>>> On 18.12.15 at 15:09, <david.vrabel@citrix.com> 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