Re: Spinlock query

["linux-os \(Dick Johnson\)" <linux-os@xxxxxxxxxxxx>]

In your driver, the critical section __disables__ interrupts,
does work in a critical section, then enables them with this:

int critical_section(struct global *params,...){
     unsigned long flags;
     spin_lock_irqsave(&params->lock, flags);
         do_critical_uninterruptible_code();
     spin_unlock_irqrestore(&params->lock flags);
  }

The interrupts are disabled on the current CPU and the flags
have been saved. The critical section of code executes without
being interrupted. In the event that another CPU tries to execute
the same code path, it will busy-wait until the spin-lock is
unlocked.



In the interrupt service routine, you do this:

irqreturn_t isr(.....) {
     spin_lock(&params->lock);
     ....
     ....
     spin_unlock(&params->lock);
     return WHATEVER;
}

Since an interrupt on the current CPU cannot occur if a spin-lock has
been taken, any other CPU will busy-wait until the lock is released.
You do not need to save/restore the flags because you know what they
are (interrupts are disabled) and they will not be changed by the
code.




Make sure you initialize any spin-locks before use. Also make sure
the spin-lock is the same one being used everywhere in your driver!
A spin-lock, private to a procedure (common newbie mistake), will not
work!

init(...)
{
     spin_lock_init(&params->lock);

}


Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.24 on an i686 machine (5592.62 BogoMips).
New book: http://www.AbominableFirebug.com/
_


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