[PATCH 3/5] mm, swap: Avoid lock swap_avail_lock when held cluster lock

["Huang, Ying" <ying.huang@intel.com>]

From: Huang Ying <ying.huang@intel.com>

Cluster lock is used to protect the swap_cluster_info and
corresponding elements in swap_info_struct->swap_map[].  But it is
found that now in scan_swap_map_slots(), swap_avail_lock may be
acquired when cluster lock is held.  This does no good except making
the locking more complex and improving the potential locking
contention, because the swap_info_struct->lock is used to protect the
data structure operated in the code already.  Fix this via moving the
corresponding operations in scan_swap_map_slots() out of cluster lock.

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Acked-by: Tim Chen <tim.c.chen@intel.com>
---
 mm/swapfile.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 42fd620dcf4c..53b5881ee0d6 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -672,6 +672,9 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
 		else
 			goto done;
 	}
+	si->swap_map[offset] = usage;
+	inc_cluster_info_page(si, si->cluster_info, offset);
+	unlock_cluster(ci);
 
 	if (offset == si->lowest_bit)
 		si->lowest_bit++;
@@ -685,9 +688,6 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
 		plist_del(&si->avail_list, &swap_avail_head);
 		spin_unlock(&swap_avail_lock);
 	}
-	si->swap_map[offset] = usage;
-	inc_cluster_info_page(si, si->cluster_info, offset);
-	unlock_cluster(ci);
 	si->cluster_next = offset + 1;
 	slots[n_ret++] = swp_entry(si->type, offset);
 
-- 
2.11.0

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