On Thu, Dec 31, 2015 at 04:09:34PM +0800, ling.ma.prog...@gmail.com wrote:
> +void alispinlock(struct ali_spinlock *lock, struct ali_spinlock_info *ali)
> +{
> + struct ali_spinlock_info *next, *old;
> +
> + ali->next = NULL;
> + ali->locked = 1;
> + old = xchg(&lock->lock_p, ali);
> +
> + /* If NULL we are the first one */
> + if (old) {
> + WRITE_ONCE(old->next, ali);
> + if(ali->flags & ALI_LOCK_FREE)
> + return;
> + while((READ_ONCE(ali->locked)))
> + cpu_relax_lowlatency();
> + return;
> + }
> + old = READ_ONCE(lock->lock_p);
> +
> + /* Handle all pending works */
> +repeat:
> + if(old == ali)
> + goto end;
> +
> + while (!(next = READ_ONCE(ali->next)))
> + cpu_relax();
> +
> + ali->fn(ali->para);
> + ali->locked = 0;
> +
> + if(old != next) {
> + while (!(ali = READ_ONCE(next->next)))
> + cpu_relax();
> + next->fn(next->para);
> + next->locked = 0;
> + goto repeat;
> +
> + } else
> + ali = next;
So I have a whole bunch of problems with this thing.. For one I object
to this being called a lock. Its much more like an async work queue like
thing.
It suffers the typical problems all those constructs do; namely it
wrecks accountability.
But here that is compounded by the fact that you inject other people's
work into 'your' lock region, thereby bloating lock hold times. Worse,
afaict (from a quick reading) there really isn't a bound on the amount
of work you inject.
This will completely wreck scheduling latency. At the very least the
callback loop should have a need_resched() test on, but even that will
not work if this has IRQs disabled.
And while its a cute collapse of an MCS lock and lockless list style
work queue (MCS after all is a lockless list), saving a few cycles from
the naive spinlock+llist implementation of the same thing, I really
do not see enough justification for any of this.
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