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