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SubjectRe: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync
On 11/23, Paul E. McKenney wrote:
>
> 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

Oh. I guess I'd better sleep before answer, but I hope that this version
doesn't have those problems. Note the 'atomic_inc_not_zero()' in read_lock(),
it seems not possible for synchronize_xxx() to return while xxx_read_lock()
increments a "wrong" element.

Just in case, in no way this interface should replace the current srcu code,
this is another variant optimized for writers, with a hope it is ok for Jens.

> 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?

From Documentation/atomic_ops.txt:

"atomic_add_unless requires explicit memory barriers around the operation."

>
> > return idx;
> > }
> > }
> >
> > void xxx_read_unlock(struct xxx_struct *sp, int idx)
> > {
>
> Need a before-atomic-dec memory barrier here?

The same, Documentation/atomic_ops.txt states

"It requires explicit memory barrier semantics"

> > 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(),

... so atomic_read() >= 0 ...

> then some other CPU executed xxx_read_unlock(), finding no one to awaken,

... it does atomic_dec(), but sp->wq is empty, yes?

> then we execute the __wait_event()?

__wait_event() will notice !atomic_read() and return.

Note that this is just an optimization. We can do

atomic_dec(sp->ctr + idx);
__wait_event(&sp->wq, !atomic_read(sp->ctr + idx));

instead. Also, I think synchronize_xxx() could be optimized further.

> What am I missing here?

Probably it is me again who missed something... Please say no!

> >
> > 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.

Thanks!

Oleg.

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