On 09/01, Peter Zijlstra wrote: > > On Mon, Sep 01, 2014 at 07:58:51PM +0200, Oleg Nesterov wrote: > > > However, the very fact that another CPU can look at this task_struct > > means that we still need spin_unlock_wait(). If nothing else to ensure > > that try_to_wake_up()->spin_unlock(pi_lock) won't write into the memory > > we are are going to free. > > task_struct is RCU freed, if it still has a 'reference' to the task,
Not really, put_task_struct() frees this memory once the counter is zero, but this doesn't matter, > it shouldn't be going 'away', right? Yes, thanks for correcting me. Somehow I forgot that the caller of ttwu() should have a reference anyway. And indeed, say, __rwsem_do_wake() does have. Otherwise this code would be obviously buggy in any case. > > So I think the comment in do exit should be updated too, and smp_mb() > > should be moved under raw_spin_unlock_wait() but ... > > > > But. If am right, doesn't this mean we that have even more problems with > > postmortem wakeups??? Why ttwu() can't _start_ after spin_unlock_wait ? > > ttwu should bail at: if (!(p->state & state)) goto out; That should > never match with TASK_DEAD. See above. I meant another problem, but I was wrong. OK. So this patch should probably work. But let me think again and send it tommorrow. Because today (and yesterday) I didn't really sleep ;) Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/