spin_lock behavior

[Jeff Haran]

From: kernelnewbies-bounces at kernelnewbies.org
[mailto:kernelnewbies-bounces at kernelnewbies.org] On Behalf Of Prajosh
Premdas
Sent: Wednesday, November 02, 2011 2:02 AM
To: kernelnewbies at kernelnewbies.org
Subject: spin_lock behavior

 

Hi 

 

I wrote a sample module which looked like this and i expected the module
to hang as i have used 2 spin_lock simultaneously. But i find the print
after the second spin_lock being printed and the module works perfectly
fine. Can any body please help me in understanding this? I got this
problem from a driver i wrote and had a typo "spin_lock" instead of
"spin_unlock" the driver worked fine and i caught the mistake only
during a review 

 

#include <linux/module.h>

#include <linux/spinlock.h>

 

struct sp_q {

spinlock_t spinlock;

uint8_t data;

};

 

static int __init sp_lck_init(void)

{

 

   struct sp_q test;

   

   /* Spin lock Init */

 

   spin_lock_init(&test.spinlock);

    

   printk("<1>Test start\n");

   

   spin_lock(&test.spinlock);

   test.data = 0;   

   spin_lock(&test.spinlock);   

   

   printk("<1>How come???\n");

   return 0;

 

}

 

static void __exit sp_lck_exit(void)

{

 

}

 

module_init(sp_lck_init);

module_exit(sp_lck_exit);

MODULE_LICENSE("GPL");

 

-- 
Regards,

Prajosh Premdas

 

On single processor systems, spin_lock() doesn't actually spin on
anything.

 

The term spin_lock() is a bit of a misnomer in my opinion as it implies
that some sort of spinning will go on if the lock has already been
taken.

 

It really means, sort of, "disable preemption, then serialize access on
this variable among other threads on other CPUs, and I promise not to
try to take this lock in any top or bottom half context on this CPU."

 

Of course, that would make for an unwieldy function name, so its call
spin_lock().

 

Jeff Haran

 


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