On 18.10.2014 23:36, Peter Zijlstra wrote: > On Sat, Oct 18, 2014 at 12:33:27PM +0400, Kirill Tkhai wrote: >> How about this? >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index b78280c..d46427e 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -1165,7 +1165,21 @@ static void task_numa_compare(struct task_numa_env >> *env, >> >> rcu_read_lock(); >> cur = ACCESS_ONCE(dst_rq->curr); >> - if (cur->pid == 0) /* idle */ >> + /* >> + * No need to move the exiting task, and this ensures that ->curr >> + * wasn't reaped and thus get_task_struct() in task_numa_assign() >> + * is safe; note that rcu_read_lock() can't protect from the final >> + * put_task_struct() after the last schedule(). >> + */ >> + if (is_idle_task(cur) || (cur->flags & PF_EXITING)) >> + cur = NULL; >> + /* >> + * Check once again to be sure curr is still on dst_rq. Even if >> + * it points on a new task, which is using the memory of freed >> + * cur, it's OK, because we've locked RCU before >> + * delayed_put_task_struct() callback is called to put its struct. >> + */ >> + if (cur != ACCESS_ONCE(dst_rq->curr)) >> cur = NULL; >> >> /* > > So you worry about the refcount doing 0->1 ? In which case the above is > still wrong and we should be using atomic_inc_not_zero() in order to > acquire the reference count. >
We can't use atomic_inc_not_zero(). The problem is that cur is pointing to a memory, which may be not a task_struct even. No guarantees at all. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
