On Fri, Jan 18, 2013 at 10:42:07AM +0800, Xiao Guangrong wrote: > On 01/17/2013 09:45 PM, Robin Holt wrote: > > On Thu, Jan 17, 2013 at 08:19:55PM +0800, Xiao Guangrong wrote: > >> On 01/17/2013 07:12 PM, Robin Holt wrote: > >>> On Thu, Jan 17, 2013 at 10:45:32AM +0800, Xiao Guangrong wrote: > >>>> On 01/17/2013 05:01 AM, Robin Holt wrote: > >>>>> > >>>>> There is a race condition between mmu_notifier_unregister() and > >>>>> __mmu_notifier_release(). > >>>>> > >>>>> Assume two tasks, one calling mmu_notifier_unregister() as a result > >>>>> of a filp_close() ->flush() callout (task A), and the other calling > >>>>> mmu_notifier_release() from an mmput() (task B). > >>>>> > >>>>> A B > >>>>> t1 srcu_read_lock() > >>>>> t2 if (!hlist_unhashed()) > >>>>> t3 srcu_read_unlock() > >>>>> t4 srcu_read_lock() > >>>>> t5 hlist_del_init_rcu() > >>>>> t6 synchronize_srcu() > >>>>> t7 srcu_read_unlock() > >>>>> t8 hlist_del_rcu() <--- NULL pointer deref. > >>>> > >>>> The detailed code here is: > >>>> hlist_del_rcu(&mn->hlist); > >>>> > >>>> Can mn be NULL? I do not think so since mn is always the embedded struct > >>>> of the caller, it be freed after calling mmu_notifier_unregister. > >>> > >>> If you look at __mmu_notifier_release() it is using hlist_del_init_rcu() > >>> which will set the hlist->pprev to NULL. When hlist_del_rcu() is called, > >>> it attempts to update *hlist->pprev = hlist->next and that is where it > >>> takes the NULL pointer deref. > >> > >> Yes, sorry for my careless. So, That can not be fixed by using > >> hlist_del_init_rcu instead? > > > > The problem is the race described above. Thread 'A' has checked to see > > if n->pprev != NULL. Based upon that, it did called the mn->release() > > method. While it was trying to call the release method, thread 'B' ended > > up calling hlist_del_init_rcu() which set n->pprev = NULL. Then thread > > 'A' got to run again and now it tries to do the hlist_del_rcu() which, as > > part of __hlist_del(), the pprev will be set to n->pprev (which is NULL) > > and then *pprev = n->next; hits the NULL pointer deref hits. > > I mean using hlist_del_init_rcu instead of hlist_del_rcu in > mmu_notifier_unregister(), hlist_del_init_rcu is aware of ->pprev. How does that address the calling of the ->release() method twice? Thanks, Robin -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>