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Message-ID: <1392381841.5384.43.camel@tkhai>
Date: Fri, 14 Feb 2014 16:44:01 +0400
From: Kirill Tkhai <ktkhai@...allels.com>
To: Catalin Marinas <catalin.marinas@....com>
CC: <linux-kernel@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>, <tkhai@...dex.ru>
Subject: Re: [PATCH] sched/core: Create new task with twice disabled
preemption
В Птн, 14/02/2014 в 12:35 +0000, Catalin Marinas пишет:
> 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:
It looks better, than the initial.
You may add my Acked-by if you want.
> 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);
> }
>
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