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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Peter Zijlstra (peterz@infradead.org) wrote:
> 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().

Cool!

Your design looks good to me. It reminds me of a latch. My only fear is
that struct timekeeper is probably too large to be copied every time on
the read path. Here is a slightly reworked version that would allow
in-place read of "foo" without copy.

struct foo {
	...
};

struct latchfoo {
	unsigned int head, tail;
	spinlock_t write_lock;
	struct foo data[2];
};

static
void foo_update(struct latchfoo *lf, void cb(struct foo *foo), void *ctx)
{
	spin_lock(&lf->write_lock);
	lf->head++;
	smp_wmb();
	lf->data[lf->head & 1] = lf->data[lf->tail & 1];
	cb(&lf->data[lf->head & 1], ctx);
	smp_wmb();
	lf->tail++;
	spin_unlock(&lock->write_lock);
}

static
unsigned int foo_read_begin(struct latchfoo *lf)
{
	unsigned int ret;

	ret = ACCESS_ONCE(lf->tail);
	smp_rmb();
	return ret;
}

static
struct foo *foo_read_get(struct latchfoo *lf, unsigned int tail)
{
	return &lf->data[tail & 1];
}

static
int foo_read_retry(struct latchfoo *lf, unsigned int tail)
{
	smp_rmb();
	return (ACCESS_ONCE(lf->head) - tail >= 2);
}

Comments are welcome,

Thanks!

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com