On 06/10/2015 04:17 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. We still need
> to do the empty list test inside the lock for safety reason, but it
> minimizes the chance of unnecessary spinlock contention.
>
> Signed-off-by: Waiman Long <waiman.l...@hp.com>
> ---
> security/selinux/hooks.c | 17 +++++++++++++----
> 1 files changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 7dade28..cd736c3 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))
> - list_del_init(&isec->list);
> - spin_unlock(&sbsec->isec_lock);
> + /*
> + * 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. Lock taking can be slow
> + * especially if the lock is being contended. We do, however, need
> + * to recheck the list again before deleting it for safety.
> + */
> + if (!list_empty(&isec->list)) {
> + spin_lock(&sbsec->isec_lock);
> + if (!list_empty(&isec->list))
> + list_del_init(&isec->list);
> + spin_unlock(&sbsec->isec_lock);
> + }
>
> /*
> * The inode may still be referenced in a path walk and
>
Do we really need the second list_empty() test at all?
Once removed, inode security structures are never re-added to the list.
For comparison, inode_sb_list_del() only tests list_empty() outside the
lock.
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