On Oct 17, 2017, at 5:40 PM, Tobin C. Harding < me@xxxxxxxx> 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_xchgthanks,Tobin._______________________________________________Kernelnewbies mailing listKernelnewbies@xxxxxxxxxxxxxxxxxhttps://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
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