atomic test and set

[Anish Kumar]

> On Oct 17, 2017, at 5:40 PM, Tobin C. Harding <me at tobin.cc> 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.
> 
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