Re: [PATCH v2] selinux: reduce locking overhead in inode_free_security()
From: Waiman Long
Date: Mon Jun 15 2015 - 12:57:42 EST
On 06/13/2015 03:35 AM, Yury wrote:
On 13.06.2015 01:35, 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.
Cheers,
Longman
Hello, Waiman!
At first, minor.
For me, moving the line 'if (!list_empty(&isec->list))' out of lock is
not possible just because 'inode_free_security' is called from
'__destroy_inode' only. You cannot rely on it in future. It's rather
possible because empty list is invariant under 'list_del_init', as you
noted here. In fact, you can call 'list_del_init' unconditionally
here, and condition is the only optimization to decrease lock
contention. So, I'd like to ask you reflect it in your comment.
I will send out an updated patch with the correct comment and commit log.
At second, less minor.
Now that you access list element outside of the lock, why don't you
use 'list_empty_careful' instead of 'list_empty'? It may eliminate
possible race between, say, 'list_add' and 'list_empty', and costs you
virtually nothing.
Best regards,
Yury
I don't think it is possible to have concurrent list_add() and
list_empty() for this particular case. However, I also don't see any
downside of using list_empty_careful() neither. So I can make the change.
Cheers,
Longman
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