Messages in this thread | | | Date | Fri, 12 Jun 2015 18:35:43 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v2] selinux: reduce locking overhead in inode_free_security() |
| |
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@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()? > 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
| |