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@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/c...
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.