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SubjectRe: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync
On Thu, Nov 23, 2006 at 05:59:10PM +0300, Oleg Nesterov wrote:
> (Sorry, responding to the wrong message)
>
> Paul E. McKenney wrote:
> >
> > I am concerned about this as well, and am beginning to suspect that I
> > need to make a special-purpose primitive specifically for Jens that he
> > can include with his code.
>
> How about this?

For Jens, it might be OK. For general use, I believe that this has
difficulties with the sequence of events I sent out on November 20th, see:
http://marc.theaimsgroup.com/?l=linux-kernel&m=116397154808901&w=2

Might also be missing a few memory barriers, see below.

> struct xxx_struct {
> int completed;
> atomic_t ctr[2];
> struct mutex mutex;
> wait_queue_head_t wq;
> };
>
> void init_xxx_struct(struct xxx_struct *sp)
> {
> sp->completed = 0;
> atomic_set(sp->ctr + 0, 1); // active
> atomic_set(sp->ctr + 1, 0); // inactive
> mutex_init(&sp->mutex);
> init_waitqueue_head(&sp->wq);
> }
>
> int xxx_read_lock(struct xxx_struct *sp)
> {
> for (;;) {
> int idx = sp->completed & 0x1;
> if (likely(atomic_inc_not_zero(sp->ctr + idx)))

Need an after-atomic-inc memory barrier here?

> return idx;
> }
> }
>
> void xxx_read_unlock(struct xxx_struct *sp, int idx)
> {

Need a before-atomic-dec memory barrier here?

> if (unlikely(atomic_dec_and_test(sp->ctr + idx)))
> wake_up(&sp->wq);
> }
>
> void synchronize_xxx(struct xxx_struct *sp)
> {
> int idx;
>
> mutex_lock(&sp->mutex);
>
> idx = ++sp->completed & 0x1;
> smp_mb__before_atomic_inc();
> atomic_inc(&sp->ctr + idx);
>
> idx = !idx;
> if (!atomic_dec_and_test(&sp->ctr + idx))
> __wait_event(&sp->wq, !atomic_read(&sp->ctr + idx));

I don't understand why an unlucky sequence of events mightn't be able
to hang this __wait_event(). Suppose we did the atomic_dec_and_test(),
then some other CPU executed xxx_read_unlock(), finding no one to awaken,
then we execute the __wait_event()? What am I missing here?

>
> mutex_unlock(&sp->mutex);
> }
>
> Yes, cache thrashing... But I think this is hard to avoid if we want writer
> to be fast.
>
> I do not claim this is the best solution, but for some reason I'd like to
> suggest something that doesn't need synchronize_sched(). What do you think
> about correctness at least?

The general approach seems reasonable, but I do have the concerns above.

Thanx, Paul
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