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@xxxxxxxx>: > > 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. _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
