Re: [PATCH v3] selinux: reduce locking overhead in inode_free_security()

[Paul Moore <paul@paul-moore.com>]

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@hp.com>
> ---
>  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