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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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