atomic test and set

[Tobin C. Harding]

On Fri, Oct 20, 2017 at 10:22:01PM +0800, Yubin Ruan wrote:
> 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 ;-)

Thanks Yubin, guess I need to work on my macro-foo

> 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

thanks,
Tobin.