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?
(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()
#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.
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