Re: spin_lock_irq / spin_lock_irqsave ??

["Bhanu Kalyan Chetlapalli" <chbhanukalyan@xxxxxxxxx>]

On 6/8/07, Bhanu Kalyan Chetlapalli <chbhanukalyan@xxxxxxxxx> wrote:

On 6/8/07, keshetti85-linux@xxxxxxxxxxx <keshetti85-linux@xxxxxxxxxxx > wrote:

> This helps prevent deadlocks where the code is potentially accessed
> from interrupt context also.

do you mean to say if I write some code like this ...

spin_lock_t lock = SPIN_LOCK_UNLOCKED ;

void some_function(void)
{
   spin_lock_irq(&lock);
    do_some_thing;
    spin_unlock_irq(&lock);
}

int intr_handler(XXXX)
{
spin_lock_irq(&lock);
do_some_thing;
spin_unlock_irq(&lock);
}
  
will it cause deadlock?

No it wont deadlock, but very bad things can happen if you dont use the irqsave & restore. Since you are NOT saving the old state of interrupt flags, you *MIGHT* end up enabling interrupts, while in fact they should have remained disabled. Check this piece of code

f1()
{
      spin_lock_irq(&lock1);
     spin_lock_irq(&lock2);
      /* Do xyz */

     spin_unlock_irq(&lock2);

       /* Do something which REQUIRES interrupts being disabled */
       /* But Interrupts are ENABLED here!! - since the unlock enabled interrupts without restoring prev state... */

       spin_unlock_irq(&lock1);
}

(This typically happens across functions, but I just wanted to make it clear)


/* Also this is the way the intr_handler() should be written in the code you cited above */

int intr_handler(XXXX)
{
/* You are ALREADY in the interrupt handler - no one can preempt you! - so dont need to disable IRQs again! */
spin_lock(&lock);
do_some_thing;
spin_unlock(&lock);
}

Just to clarify, If the data structure you are protecting is accessed from different handlers, then you DO need to use the spin_lock_saveirq etc.

Elaborating further, This is the hierarchy of preemptability in the kernel

An entry in this list can preempt any one below, but not above it.

Interrupt Handlers
Soft IRQs/Tasklets
Process Context

So if you are protecting a DS in user context, which is also used by a Tasklet, use the spin_lock_bh()

> Spinlocks are used in the kernel only, so I kinda missed the point here.
> Can you rephrase?

Actually I thought that spin_lock_irqsave will save the total context.
I understood it now.

-Mahesh.

Bhanu

--
The box said "Requires Windows Vista or better." So I installed LINUX

-- 
The box said "Requires Windows Vista or better." So I installed LINUX