[PATCH] xen/blkfront: speed up purge_persistent_grants()

[Juergen Gross]

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@xxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 drivers/block/xen-blkfront.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index aa996b637d0b..378262ec47ae 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -2533,6 +2533,7 @@ 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);
 
@@ -2550,9 +2551,11 @@ static void purge_persistent_grants(struct blkfront_info *info)
 			list_del(&gnt_list_entry->node);
 			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);
 	}
 }
-- 
2.34.1