atomic test and set

Tobin C. Harding me at
Tue Oct 17 20:40:44 EDT 2017


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


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