[PATCH] dm snapshot: tidy pe ref counting

	/*

	 * The primary pending_exception is the one that holds

	 * the sibling_count and the list of origin_bios for a

	 * the ref_count and the list of origin_bios for a

	 * group of pending_exceptions.  It is always last to get freed.

	 * These fields get set up when writing to the origin.

	 */

	 * the sibling concerned and not pe->primary_pe->snap->lock unless

	 * they are the same.

	 */

	atomic_t sibling_count;

	atomic_t ref_count;

	/* Pointer back to snapshot context */

	struct dm_snapshot *snap;

	dm_table_event(s->table);

}

static void get_pending_exception(struct pending_exception *pe)

{

	atomic_inc(&pe->ref_count);

}

static struct bio *put_pending_exception(struct pending_exception *pe)

{

	struct pending_exception *primary_pe;

	struct bio *origin_bios = NULL;

	primary_pe = pe->primary_pe;

	/*

	 * If this pe is involved in a write to the origin and

	 * it is the last sibling to complete then release

	 * the bios for the original write to the origin.

	 */

	if (primary_pe &&

	    atomic_dec_and_test(&primary_pe->ref_count))

		origin_bios = bio_list_get(&primary_pe->origin_bios);

	/*

	 * Free the pe if it's not linked to an origin write or if

	 * it's not itself a primary pe.

	 */

	if (!primary_pe || primary_pe != pe)

		free_pending_exception(pe);

	/*

	 * Free the primary pe if nothing references it.

	 */

	if (primary_pe && !atomic_read(&primary_pe->ref_count))

		free_pending_exception(primary_pe);

	return origin_bios;

}

static void pending_complete(struct pending_exception *pe, int success)

{

	struct exception *e;

	struct pending_exception *primary_pe;

	struct dm_snapshot *s = pe->snap;

	struct bio *origin_bios = NULL;

	struct bio *snapshot_bios = NULL;

 out:

	snapshot_bios = bio_list_get(&pe->snapshot_bios);

	primary_pe = pe->primary_pe;

	/*

	 * If this pe is involved in a write to the origin and

	 * it is the last sibling to complete then release

	 * the bios for the original write to the origin.

	 */

	if (primary_pe &&

	    atomic_dec_and_test(&primary_pe->sibling_count))

		origin_bios = bio_list_get(&primary_pe->origin_bios);

	/*

	 * Free the pe if it's not linked to an origin write or if

	 * it's not itself a primary pe.

	 */

	if (!primary_pe || primary_pe != pe)

		free_pending_exception(pe);

	/*

	 * Free the primary pe if nothing references it.

	 */

	if (primary_pe && !atomic_read(&primary_pe->sibling_count))

		free_pending_exception(primary_pe);

	origin_bios = put_pending_exception(pe);

	up_write(&s->lock);

	bio_list_init(&pe->origin_bios);

	bio_list_init(&pe->snapshot_bios);

	pe->primary_pe = NULL;

	atomic_set(&pe->sibling_count, 1);

	atomic_set(&pe->ref_count, 0);

	pe->snap = s;

	pe->started = 0;

		return NULL;

	}

	get_pending_exception(pe);

	insert_exception(&s->pending, &pe->e);

 out:

		 * is already remapped in this snapshot

		 * and trigger an exception if not.

		 *

		 * sibling_count is initialised to 1 so pending_complete()

		 * ref_count is initialised to 1 so pending_complete()

		 * won't destroy the primary_pe while we're inside this loop.

		 */

		e = lookup_exception(&snap->complete, chunk);

		}

		if (!pe->primary_pe) {

			atomic_inc(&primary_pe->sibling_count);

			pe->primary_pe = primary_pe;

			get_pending_exception(primary_pe);

		}

		if (!pe->started) {

	}

	if (!primary_pe)

		goto out;

		return r;

	/*

	 * If this is the first time we're processing this chunk and

	 * sibling_count is now 1 it means all the pending exceptions

	 * ref_count is now 1 it means all the pending exceptions

	 * got completed while we were in the loop above, so it falls to

	 * us here to remove the primary_pe and submit any origin_bios.

	 */

	if (first && atomic_dec_and_test(&primary_pe->sibling_count)) {

	if (first && atomic_dec_and_test(&primary_pe->ref_count)) {

		flush_bios(bio_list_get(&primary_pe->origin_bios));

		free_pending_exception(primary_pe);

		/* If we got here, pe_queue is necessarily empty. */

		goto out;

		return r;

	}

	/*

	list_for_each_entry_safe(pe, next_pe, &pe_queue, list)

		start_copy(pe);

 out:

	return r;

}