atomic test and set

[Yubin Ruan]

On Wed, Oct 18, 2017 at 11:40:44AM +1100, Tobin C. Harding wrote:
> Hi,
> 
> Do we have an atomic test and set function in the kernel. I have tried
> 
> const int KEY_FLAG_BIT = 1;
> 
> ...
> 
> 	static siphash_key_t ptr_secret __read_mostly;
> 	static unsigned long have_key = 0;
> 
> 	if (test_and_set_bit(KEY_FLAG_BIT, &have_key))
> 		get_random_bytes(&ptr_secret, sizeof(ptr_secret));
> 
> 
> But that doesn't work.
> 
> I looked in include/linux/atomic.h and thought about using
> 
> 	static atomic_t have_key = ATOMIC_INIT(0);
> 
>         if (atomic_xchg(&have_key, 1) == 0)
>         	get_random_bytes(&ptr_secret, sizeof(ptr_secret));
> 
> 
> This works. My question is; does this code LOAD the value at have_key and STORE the argument on
> every call? Or does it LOAD the value, check if it is the same as the argument, and STORE _only_ if
> it is different? 

Yes it stores at every call. To check before load, you should use cmpxchg
 
> (Is this whole discussion just premature optimization?)
> 
> I cannot grok the macros in atomic.h, they seem circular. Here is the macro definitions in call
> chain order starting with atomic_xchg()

I think those macros look loverly ;-)
Maybe you should look into `Documentation/atomic_ops.txt' for help. Also, I
think see how xchg() and cmpxchg() is implemented in the kernel, e.g., at here:

    https://elixir.free-electrons.com/linux/latest/source/arch/x86/include/asm/cmpxchg.h

Yubin