PPC arch and spinlocks

[Hollis Blanchard]

On Jun 8, 2005, at 9:29 AM, Garcia Jérémie wrote:
> I'm new to kernel device driver (linux newbie) and I'd like to control 
> concurrent access
> to our hardware resources. (I'm working on a ppc405EP platform 
> uniprocessor)
> ...
> So, I decided to declare a spinlock in my module and offer, via ioctl, 
> the possibilty to user
> space programs to handle it with spin_trylock(), spin_lock() and 
> spin_unlock().

I don't think this makes any sense. Kernel spinlocks are to protect 
critical sections of kernel code from race conditions.

> I really need to have an equivalent of the spin_trylock() routine in 
> order not to have the processus waiting till the semaphore is 
> available when tryin to get it. (ask and take if available, but not 
> sleep).

It sounds like what you want is to enforce only a single user of your 
device driver. If that's the case, you will need an atomic operation, 
but if that fails then simply return an appropriate error code from 
your open() routine.

You could use a spinlock like drivers/char/nvram.c:
static int
nvram_open(struct inode *inode, struct file *file)
{
     spin_lock(&nvram_state_lock);
     if ((nvram_open_cnt && (file->f_flags & O_EXCL)) || ...)
         spin_unlock(&nvram_state_lock);
         return -EBUSY;
     }

     ...
     nvram_open_cnt++;
     spin_unlock(&nvram_state_lock);
     return 0;
}

In this example, "nvram_open_cnt++" is one of the operations being 
protected from race conditions, such as two processes on an SMP system 
both entering open() at literally the same time. Of course, since you 
aren't building SMP, you know that there will not be two processes in 
open() simultaneously, so the spinlocks will be compiled out, and you 
will be left with a plain counter returning -EBUSY if the device is 
already in use.

-Hollis