Re: [PATCH] sched/core: Create new task with twice disabled preemption

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Feb 14, 2014 at 12:44:01PM +0000, Kirill Tkhai wrote:
> 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.

Thanks for the ack. But apart from arm64, are there any other problems
with running part of finish_task_switch() and post_schedule() with
preemption enabled?

The finish_arch_post_lock_switch() is currently only used by arm and
arm64 (the former UP only) and arm no longer has the preemption problem
(see commit bdae73cd374e2). So I can either disable the preemption
around schedule_tail() call in arm64 or do it globally as per yours or
my patch.

Peter, Ingo, any thoughts? Do we care about preempt count consistency
across finish_task_switch() and post_schedule()?

Thanks.

-- 
Catalin