On Monday, June 15, 2015 01:13:39 PM 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. The list_del_init()
> can be called more than once on the same list with no harm as long
> as they are properly serialized. It should not be possible to have
> inode_free_security() called concurrently with list_add(). For better
> safety, however, we use list_empty_careful() here even though it is
> still not completely safe in case that happens.
>
> Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
> ---
> security/selinux/hooks.c | 17 ++++++++++++++---
> 1 files changed, 14 insertions(+), 3 deletions(-)
>
> v1->v2:
> - Take out the second list_empty() test inside the lock.
>
> v2->v3:
> - Fix incorrent comment and commit log message.
Thanks for the patch and the discussion; I've added this to the SELinux next-
queue branch and I'll push it to selinux#next as soon as the merge window
closes.
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 7dade28..2a99804 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -254,10 +254,21 @@ 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.
> + *
> + * The list_del_init() function can be safely called more than once.
> + * It should not be possible for this function to be called with
> + * concurrent list_add(), but for better safety against future changes
> + * in the code, we use list_empty_careful() here.
> + */
> + if (!list_empty_careful(&isec->list)) {
> + spin_lock(&sbsec->isec_lock);
> list_del_init(&isec->list);
> - spin_unlock(&sbsec->isec_lock);
> + spin_unlock(&sbsec->isec_lock);
> + }
>
> /*
> * The inode may still be referenced in a path walk and
--
paul moore
www.paul-moore.com
_______________________________________________
Selinux mailing list
Selinux@xxxxxxxxxxxxx
To unsubscribe, send email to Selinux-leave@xxxxxxxxxxxxx.
To get help, send an email containing "help" to Selinux-request@xxxxxxxxxxxxx.
