> On Oct 17, 2017, at 5:40 PM, Tobin C. Harding <[email protected]> wrote: > > Hi, > > Do we have an atomic test and set function in the kernel. I have tried
Yes and it is used very much inside the kernel. > > 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. What doesn't work? What you expected and how is the result different? > > 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? Did you read this https://www.mjmwired.net/kernel/Documentation/atomic_ops.txt ? > (Is this whole discussion just premature optimization?) This has nothing to do with optimization but rather the requirement. In kernel atomic variables are used to avoid race conditions generally. > > I cannot grok the macros in atomic.h, they seem circular. Here is the macro > definitions in call > chain order starting with atomic_xchg() Implementation would differ based on the architecture. In some architectures it would be a single instruction and in others it may not be the case. > > > #define atomic_xchg(...) \ > __atomic_op_fence(atomic_xchg, __VA_ARGS__) > > #define __atomic_op_fence(op, args...) \ > ({ \ > typeof(op##_relaxed(args)) __ret; \ > smp_mb__before_atomic(); \ > __ret = op##_relaxed(args); \ > smp_mb__after_atomic(); \ > __ret; \ > }) > > #define atomic_xchg_release(...) \ > __atomic_op_release(atomic_xchg, __VA_ARGS__) > > #define __atomic_op_release(op, args...) \ > ({ \ > smp_mb__before_atomic(); \ > op##_relaxed(args); \ > }) > > #define atomic_xchg_relaxed atomic_xchg > > thanks, > Tobin. > > _______________________________________________ > Kernelnewbies mailing list > [email protected] > https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
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