Messages in this thread |  | | | Date | Thu, 23 Nov 2006 12:40:54 -0800 | | From | "Paul E. McKenney" <> | | Subject | Re: [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.
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