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
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