On Fri, 18 Nov 2011, Miao Xie wrote: > >> I find these is another problem, please take account of the following case: > >> > >> 2-3 -> 1-2 -> 0-1 > >> > >> the user change mems_allowed twice continuously, the task may see the empty > >> mems_allowed. > >> > >> So, it is still dangerous. > >> > > > > With this patch, we're protected by task_lock(tsk) to determine whether we > > want to take the exception, i.e. whether need_loop is false, and the > > setting of tsk->mems_allowed. So this would see the nodemask change at > > the individual steps from 2-3 -> 1-2 -> 0-1, not some inconsistent state > > in between or directly from 2-3 -> 0-1. The only time we don't hold > > task_lock(tsk) to change tsk->mems_allowed is when tsk == current and in > > that case we're not concerned about intermediate reads to its own nodemask > > while storing to a mask where MAX_NUMNODES > BITS_PER_LONG. > > > > Thus, there's no problem here with regard to such behavior if we exclude > > mempolicies, which this patch does. > > > > No. > When the task does memory allocation, it access its mems_allowed without > task_lock(tsk), and it may be blocked after it check 0-1 bits. And then, the > user changes mems_allowed twice continuously(2-3(initial state) -> 1-2 -> 0-1), > After that, the task is woke up and it see the empty mems_allowed. > I'm confused, you're concerned on a kernel where MAX_NUMNODES > BITS_PER_LONG about thread A reading a partial tsk->mems_allowed, being preempted, meanwhile thread B changes tsk->mems_allowed by taking cgroup_mutex, taking task_lock(tsk), setting the intersecting nodemask, releasing both, taking them again, changing the nodemask again to be disjoint, then the thread A waking up and finishing its read and seeing an intersecting nodemask because it is now disjoint from the first read? -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>