Re: [PATCH 05/10] writeback: support > 1 flusher thread per bdi

[Jamie Lokier <jamie@shareable.org>]

Artem Bityutskiy wrote:
> Jens Axboe wrote:
> >+static void bdi_queue_work(struct backing_dev_info *bdi, struct bdi_work 
> >*work)
> >+{
> >+	if (work) {
> >+		work->seen = bdi->wb_mask;
> >+		BUG_ON(!work->seen);
> >+		atomic_set(&work->pending, bdi->wb_cnt);
> >+		BUG_ON(!bdi->wb_cnt);
> >+
> >+		/*
> >+		 * Make sure stores are seen before it appears on the list
> >+		 */
> >+		smp_mb();
> >+
> >+		spin_lock(&bdi->wb_lock);
> >+		list_add_tail_rcu(&work->list, &bdi->work_list);
> >+		spin_unlock(&bdi->wb_lock);
> >+	}
> 
> Doesn't spin_lock() include an implicit memory barrier?
> After &bdi->wb_lock is acquired, it is guaranteed that all
> memory operations are finished.

I'm pretty sure spin_lock() is an "acquire" barrier, which just guarantees
loads/stores after the spin_lock() are done after taking the lock.

It doesn't guarantee anything about loads/stores before the spin_lock().

-- Jamie