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. -- 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>