How spin_lock and preempt disable works?

["Manjunath Naik" <peter.desouzain@xxxxxxxxx>]

Hi all,

 

I was going through spin_lock macro.  This method uses the  following code.

 

I was not able to understand what exaclty is happened in this macro.

 

Spin_lock diables the preempting of the current thread. For that I think it uses thread_info structure which has preempt_count.

 

But I was not able to understand how exactly it works:

 

The Macro Flow looks like as follows for me:

 

# define add_preempt_count(val) do { preempt_count() += (val); } while (0)
# define sub_preempt_count(val) do { preempt_count() -= (val); } while (0)
 

#define inc_preempt_count() add_preempt_count(1)

 

#define preempt_disable() \
do { \
 inc_preempt_count(); \
 barrier(); \
} while (0)
 

#define __LOCK(lock) \
  do { preempt_disable(); __acquire(lock); (void)(lock); } while (0)
 

#define _spin_lock(lock)   __LOCK(lock)

 

#define spin_lock(lock)   _spin_lock(lock)
 

But I was not able to understand how exactly preempting works. when schedular wants to schedule the other process, it checks the thread_info->preempt_count value? If it is yes plese provide me some links or code reference for it.

 

 

Thanks

Manjunath