xen/blkfront: speed up purge_persistent_grants()

purge_persistent_grants() is scanning the grants list for persistent
grants being no longer in use by the backend. When having found such a
grant, it will be set to "invalid" and pushed to the tail of the list.

Instead of pushing it directly to the end of the list, add it first to
a temporary list, avoiding to scan those entries again in the main
list traversal. After having finished the scan, append the temporary
list to the grant list.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Link: https://lore.kernel.org/r/20220311103527.12931-1-jgross@suse.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/xen-blkfront.c

@@ -2521,6 +2521,7 @@ static void purge_persistent_grants(struct blkfront_info *info)
 
 	for_each_rinfo(info, rinfo, i) {
 		struct grant *gnt_list_entry, *tmp;
+		LIST_HEAD(grants);
 
 		spin_lock_irqsave(&rinfo->ring_lock, flags);
 
@@ -2539,9 +2540,11 @@ static void purge_persistent_grants(struct blkfront_info *info)
 			gnttab_end_foreign_access(gnt_list_entry->gref, 0, 0UL);
 			rinfo->persistent_gnts_c--;
 			gnt_list_entry->gref = GRANT_INVALID_REF;
-			list_add_tail(&gnt_list_entry->node, &rinfo->grants);
+			list_add_tail(&gnt_list_entry->node, &grants);
 		}
 
+		list_splice_tail(&grants, &rinfo->grants);
+
 		spin_unlock_irqrestore(&rinfo->ring_lock, flags);
 	}
 }