Re: [PATCH v2] selinux: reduce locking overhead in inode_free_security()

From: Raghavendra K T
Date: Sun Jun 14 2015 - 00:20:16 EST


On 06/13/2015 04:05 AM, Waiman Long wrote:
On 06/12/2015 08:31 AM, Stephen Smalley wrote:
On 06/12/2015 02:26 AM, Raghavendra K T wrote:
On 06/12/2015 03:01 AM, Waiman Long wrote:
The inode_free_security() function just took the superblock's isec_lock
before checking and trying to remove the inode security struct from the
linked list. In many cases, the list was empty and so the lock taking
is wasteful as no useful work is done. On multi-socket systems with
a large number of CPUs, there can also be a fair amount of spinlock
contention on the isec_lock if many tasks are exiting at the same time.

This patch changes the code to check the state of the list first
before taking the lock and attempting to dequeue it. As this function
is called indirectly from __destroy_inode(), there can't be another
instance of inode_free_security() running on the same inode.

Signed-off-by: Waiman Long<Waiman.Long@xxxxxx>
---
security/selinux/hooks.c | 15 ++++++++++++---
1 files changed, 12 insertions(+), 3 deletions(-)

v1->v2:
- Take out the second list_empty() test inside the lock.

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 7dade28..e5cdad7 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -254,10 +254,19 @@ static void inode_free_security(struct inode
*inode)
struct inode_security_struct *isec = inode->i_security;
struct superblock_security_struct *sbsec =
inode->i_sb->s_security;

- spin_lock(&sbsec->isec_lock);
- if (!list_empty(&isec->list))
+ /*
+ * As not all inode security structures are in a list, we check
for
+ * empty list outside of the lock to make sure that we won't waste
+ * time taking a lock doing nothing. As inode_free_security() is
+ * being called indirectly from __destroy_inode(), there is no way
+ * there can be two or more concurrent calls. So doing the
list_empty()
+ * test outside the loop should be safe.
+ */
+ if (!list_empty(&isec->list)) {
+ spin_lock(&sbsec->isec_lock);
list_del_init(&isec->list);
Stupid question,

I need to take a look at list_del_init() code, but it can so happen that
if !list_empty() check could happen simultaneously, then serially two
list_del_init() can happen.

is that not a problem()?
Hmm...I suppose that's possible (sb_finish_set_opts and
inode_free_security could both perform the list_del_init). Ok, we'll
stay with the first version.


Actually, list_del_init() can be applied twice with no harm being done.
The first list_del_init() will set list-> next = list->prev = list. The
second one will do the same thing and so it should be safe.


Waiman,
I do not think it is just about list_del_init() twice

what if


CPU1 CPU2 CPU3

!list_empty() !list_empty()

lock
list_del_init()
unlock

list_add()
lock
list_del_init
unlock

But this is valid only if list_add() is possible after first
list_del_init. I need to see code though.
OR am I missing something?












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