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

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

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@hp.com>
> ---
>   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()?

> -	spin_unlock(&sbsec->isec_lock);
> +		spin_unlock(&sbsec->isec_lock);
> +	}
>
>   	/*
>   	 * The inode may still be referenced in a path walk and
>