Re: test_and_set_bit implementation

[Zoltan Menyhart <Zoltan.Menyhart@bull.net>]

Christoph Lameter wrote:

> I think this will work. However, usually you execute bit test and test 
> because the bit is not set and you want to set it. This change would 
> optimize a rarely taken path. Probably not worth it.

How much is the probability that the bit is not set?

Adding a test can cost only a few cycles, say max 4.

For an atomic operation, you need to go out and snoop.

Let's have a look at e.g. the "bit_spin_lock()":

static inline void bit_spin_lock(int bitnum, unsigned long *addr)
{
        preempt_disable();
        while (test_and_set_bit(bitnum, addr)) {
                while (test_bit(bitnum, addr)) {
                        preempt_enable();
                        cpu_relax();
                        preempt_disable();
                }
        }
        __acquire(bitlock);
}

By executing the atomic operation unconditionally, you kill
the cache line all the other waiting processors looping at.

Thanks,

Zoltán Menyhárt