atomic test and set

[Yubin Ruan]

2017-10-20 16:55 GMT+08:00 Tobin C. Harding <me at tobin.cc>:
> On Fri, Oct 20, 2017 at 10:22:01PM +0800, Yubin Ruan wrote:
>> On Wed, Oct 18, 2017 at 11:40:44AM +1100, Tobin C. Harding wrote:
>> > 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?
>>
>> Yes it stores at every call. To check before load, you should use cmpxchg

sorry this should be "check before store" ...

>>
>> > (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()
>>
>> I think those macros look loverly ;-)
>
> Thanks Yubin, guess I need to work on my macro-foo
>
>> Maybe you should look into `Documentation/atomic_ops.txt' for help. Also, I
>> think see how xchg() and cmpxchg() is implemented in the kernel, e.g., at here:
>>
>>     https://elixir.free-electrons.com/linux/latest/source/arch/x86/include/asm/cmpxchg.h

I really think the `cmpxchg.h' is valuable for learning. Please do
take a look at it.

And sorry again for so many typos.

Yubin