dm kcopyd: remove superfluous page allocation spinlock

Remove the spinlock protecting the pages allocation.  The spinlock is only
taken on initialization or from single-threaded workqueue.  Therefore, the
spinlock is useless.

The spinlock is taken in kcopyd_get_pages and kcopyd_put_pages.

kcopyd_get_pages is only called from run_pages_job, which is only
called from process_jobs called from do_work.

kcopyd_put_pages is called from client_alloc_pages (which is initialization
function) or from run_complete_job. run_complete_job is only called from
process_jobs called from do_work.

Another spinlock, kc->job_lock is taken each time someone pushes or pops
some work for the worker thread.  Once we take kc->job_lock, we
guarantee that any written memory is visible to the other CPUs.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-kcopyd.c

@@ -36,7 +36,6 @@
  * pages for kcopyd io.
  *---------------------------------------------------------------*/
 struct dm_kcopyd_client {
-	spinlock_t lock;
 	struct page_list *pages;
 	unsigned int nr_pages;
 	unsigned int nr_free_pages;
@@ -99,11 +98,8 @@ static int kcopyd_get_pages(struct dm_kcopyd_client *kc,
 {
 	struct page_list *pl;
 
-	spin_lock(&kc->lock);
-	if (kc->nr_free_pages < nr) {
-		spin_unlock(&kc->lock);
+	if (kc->nr_free_pages < nr)
 		return -ENOMEM;
-	}
 
 	kc->nr_free_pages -= nr;
 	for (*pages = pl = kc->pages; --nr; pl = pl->next)
@@ -112,23 +108,19 @@ static int kcopyd_get_pages(struct dm_kcopyd_client *kc,
 	kc->pages = pl->next;
 	pl->next = NULL;
 
-	spin_unlock(&kc->lock);
-
 	return 0;
 }
 
 static void kcopyd_put_pages(struct dm_kcopyd_client *kc, struct page_list *pl)
 {
 	struct page_list *cursor;
 
-	spin_lock(&kc->lock);
 	for (cursor = pl; cursor->next; cursor = cursor->next)
 		kc->nr_free_pages++;
 
 	kc->nr_free_pages++;
 	cursor->next = kc->pages;
 	kc->pages = pl;
-	spin_unlock(&kc->lock);
 }
 
 /*
@@ -625,7 +617,6 @@ int dm_kcopyd_client_create(unsigned int nr_pages,
 	if (!kc)
 		return -ENOMEM;
 
-	spin_lock_init(&kc->lock);
 	spin_lock_init(&kc->job_lock);
 	INIT_LIST_HEAD(&kc->complete_jobs);
 	INIT_LIST_HEAD(&kc->io_jobs);