linux-kernel - [PATCH 3/5] autofs4: don't take spinlock when not needed in autofs4_lookup

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Date:	Fri, 18 Jul 2014 10:48:28 +1000
From:	NeilBrown <neilb@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	autofs@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ian Kent <raven@...maw.net>
Subject: [PATCH 3/5] autofs4: don't take spinlock when not needed in
 autofs4_lookup_expiring

If the expiring_list is empty, we can avoid a costly spinlock
in the rcu-walk path through autofs4_d_manage (once the rest
of the path becomes rcu-walk friendly).

Signed-off-by: NeilBrown <neilb@...e.de>
Reviewed-by: Ian Kent <raven@...maw.net>
---
 fs/autofs4/root.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
index cc87c1abac97..fb202cadd4b3 100644
--- a/fs/autofs4/root.c
+++ b/fs/autofs4/root.c
@@ -166,8 +166,10 @@ static struct dentry *autofs4_lookup_active(struct dentry *dentry)
 	const unsigned char *str = name->name;
 	struct list_head *p, *head;
 
-	spin_lock(&sbi->lookup_lock);
 	head = &sbi->active_list;
+	if (list_empty(head))
+		return NULL;
+	spin_lock(&sbi->lookup_lock);
 	list_for_each(p, head) {
 		struct autofs_info *ino;
 		struct dentry *active;
@@ -218,8 +220,10 @@ static struct dentry *autofs4_lookup_expiring(struct dentry *dentry)
 	const unsigned char *str = name->name;
 	struct list_head *p, *head;
 
-	spin_lock(&sbi->lookup_lock);
 	head = &sbi->expiring_list;
+	if (list_empty(head))
+		return NULL;
+	spin_lock(&sbi->lookup_lock);
 	list_for_each(p, head) {
 		struct autofs_info *ino;
 		struct dentry *expiring;


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