Re: [RFC PATCH] timekeeping: introduce timekeeping_is_busy()

Peter Zijlstra
2013-09-12T01:25:51Z

On Wed, Sep 11, 2013 at 08:48:11PM -0400, Mathieu Desnoyers wrote:
> Thoughts ?

struct foo {
...
};

spinlock_t foo_lock;
unsigned int foo_head = 0;
unsigned int foo_tail = 0;
struct foo foo_array[2];

void foo_assign(struct foo f)
{
spin_lock(&foo_lock);
foo_head++;
smp_wmb();
foo_array[foo_head & 1] = f;
smp_wmb();
foo_tail++;
spin_unlock(&foo_lock);
}

struct foo foo_get(void)
{
unsigned int tail, head;
struct foo ret;

again:
tail = ACCESS_ONCE(foo_tail);
smp_rmb();
ret = foo_array[tail & 1];
smp_rmb();
head = ACCESS_ONCE(foo_head);
if (head - tail >= 2)
goto again;

return ret;
}

Should work and get you the most recent 'complete' foo even when
foo_get() is called nested inside foo_assign().