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

Mathieu Desnoyers
2013-09-12T03:22:56Z

* 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.