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? > >> >>> >>> Tested with this patch applied. My test case which was failing >>> approximately every 300th iteration passed 25,000 tests. >> >> Could you please share your test case? > > I could but it would be very useless. It depends upon having a SGI > UV system with GRUs and and xpmem kernel module loaded. If you would > really like all the bits, I could provide them, but you will not be able > to reproduce the failure. Oh, i see. :) -- 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>