Messages in this thread | | | Date | Sun, 14 Jun 2015 09:31:11 +0530 | From | Raghavendra K T <> | Subject | Re: [PATCH v2] selinux: reduce locking overhead in inode_free_security() |
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On 06/13/2015 04:05 AM, Waiman Long wrote: > 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. >
Waiman, I do not think it is just about list_del_init() twice
what if
CPU1 CPU2 CPU3
!list_empty() !list_empty()
lock list_del_init() unlock
list_add() lock list_del_init unlock
But this is valid only if list_add() is possible after first list_del_init. I need to see code though. OR am I missing something?
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