Spinlocks and interrupts

[Jeff Haran]

> -----Original Message-----
> From: kernelnewbies-bounces+jharan=bytemobile.com at kernelnewbies.org
> [mailto:kernelnewbies-
> bounces+jharan=bytemobile.com at kernelnewbies.org] On Behalf Of Kai
> Meyer
> Sent: Wednesday, November 09, 2011 3:12 PM
> To: kernelnewbies at kernelnewbies.org
> Subject: Re: Spinlocks and interrupts
> 
> Ok, I need mutual exclusion on a data structure regardless of
interrupts
> and core. It sounds like it can be done by using a spinlock and
> disabling interrupts, but you mention that "spinlocks are intended to
> provide mutual exclsion between interrupt context and non-interrupt
> context." Should I be using a semaphore (mutex) instead?
> 
> Perhaps I could explain my problem with some code:
> struct my_struct *get_data(spinlock_t *mylock, int ALLOC_DATA)
> {
>      struct my_struct *mydata = NULL;
>      spin_lock(mylock);
>      if (test_bit(index, mybitmap))
>              mydata = retrieve_data();
>      if (!mydata && ALLOC_DATA) {
>              mydata = alloc_data();
>              set_bit(index, mybitmap);
>      }
>      spin_unlock(mylock);
>      return mydata;
> }
> 
> I need to prevent retrieve_data from being called if the index bit is
> set in mybitmap and alloc_data has not completed, so I use a bitmap to
> indicate that alloc_data has completed. I also need to protect
> alloc_data from being run multiple times, so I use the spin_lock to
> ensure that test_bit (and possibly retrieve_data) is not run while
> alloc_data is being run (because it runs while the bit is cleared).
> 
> -Kai Meyer

You probably want to lock with spin_lock_irqsave() and unlock with
spin_unlock_irqrestore() if you are not sure about what contexts
get_data() will be called in.

There's plenty of examples of how to use these in the kernel sources.

I note you are passing in the address of the spinlock itself. Be wary of
deadly embraces where two threads acquire two locks in different order.

Jeff Haran