On Thu, Feb 13, 2014 at 07:51:56PM +0400, Kirill Tkhai wrote:
> Preemption state on enter in finish_task_switch() is different
> in cases of context_switch() and schedule_tail().
>
> In the first case we have it twice disabled: at the start of
> schedule() and during spin locking. In the second it is only
> once: the value which was set in init_task_preempt_count().
>
> For archs without __ARCH_WANT_UNLOCKED_CTXSW set this means
> that all newly created tasks execute finish_arch_post_lock_switch()
> and post_schedule() with preemption enabled.
>
> It seems there is possible a problem in rare situations on arm64,
> when one freshly created thread preempts another before
> finish_arch_post_lock_switch() has finished. If mm is the same,
> then TIF_SWITCH_MM on the second won't be set.
>
> The second rare but possible issue is zeroing of post_schedule()
> on a wrong cpu.
>
> So, lets fix this and unify preempt_count state.
An alternative to your patch:
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b46131ef6aab..b932b6a4c716 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2210,6 +2210,10 @@ asmlinkage void schedule_tail(struct task_struct *prev)
{
struct rq *rq = this_rq();
+#ifndef __ARCH_WANT_UNLOCKED_CTXSW
+ /* In this case, finish_task_switch reenables preemption */
+ preempt_disable();
+#endif
finish_task_switch(rq, prev);
/*
@@ -2218,10 +2222,7 @@ asmlinkage void schedule_tail(struct task_struct *prev)
*/
post_schedule(rq);
-#ifdef __ARCH_WANT_UNLOCKED_CTXSW
- /* In this case, finish_task_switch does not reenable preemption */
preempt_enable();
-#endif
if (current->set_child_tid)
put_user(task_pid_vnr(current), current->set_child_tid);
}
--
Catalin
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