On 06/23, Linus Torvalds wrote: > > On Thu, Jun 23, 2016 at 10:03 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote: > > > > Let me quote my previous email ;) > > > > And we can't free/nullify it when the parent/debuger reaps a zombie, > > say, mark_oom_victim() expects that get_task_struct() protects > > thread_info as well. > > > > probably we can fix all such users though... > > TIF_MEMDIE is indeed a potential problem, but I don't think > mark_oom_victim() is actually problematic. > > mark_oom_victim() is called with either "current", This is no longer true in -mm tree. But I agree, this is fixable (and in fact I still hope TIF_MEMDIE will die, at least in its current form). But I am afraid we can have more users which assume that thread_info can't go away if you have a reference to task_struct. And yes, we have a users which rely on RCU, say show_state_filter() which walks the task under rcu_read_lock() and calls sched_show_task() which prints task_thread_info(p)->flags. Yes this is fixable too, but > so these days, thread_info has almost nothing really critical in it > any more. There's the thread-local flags, yes, but they could stay or > easily be moved to the task_struct or get similar per-cpu fixup as > preempt_count did a couple of years ago. The only annoyance is the few > remaining entry code assembly sequences, but I suspect they would > actually become simpler with a per-cpu thing, and with Andy's cleanups > they are pretty insignificant these days. There seems to be exactly > two uses of ASM_THREAD_INFO(TI_flags,.. left. So perhaps on x86_64 we should move thread_info from thread_union to task_struct->thread as Andy suggests. And just in case, even if we move thread_info, of course we will need to change dump_trace/etc which reads ->stack. Again, show_state_filter() relies on RCU, proc_pid_stack() on get_task_struct(). They need to pin task->stack somehow, but this is clear. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
