atomic test and set

[Tobin C. Harding]

On Fri, Oct 20, 2017 at 05:33:19PM +0800, Yubin Ruan wrote:
> 2017-10-20 16:55 GMT+08:00 Tobin C. Harding <me at tobin.cc>:
> > 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
> 
> sorry this should be "check before store" ...
> 
> >>
> >> > (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
> 
> I really think the `cmpxchg.h' is valuable for learning. Please do
> take a look at it.

I read atomic_ops.txt, thanks. Also I used cmpxchg() like this 

static atomic_t foo = ATOMIC_INIT(0);

...
        if (atomic_cmpxchg(&foo, 0, 1) == 0) {
                do_this();
        }

With the intent that only a single caller will ever call `do_this()`.

thanks,
Tobin.