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Date:   Mon, 10 Dec 2018 17:30:09 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Daniel Vetter <daniel.vetter@ffwll.ch>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
        =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
        Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [PATCH 2/4] kernel.h: Add non_block_start/end()
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On Mon, Dec 10, 2018 at 05:20:10PM +0100, Michal Hocko wrote:
> > OK, no real objections to the thing.  Just so long we're all on the same
> > page as to what it does and doesn't do ;-)
> 
> I am not really sure whether there are other potential users besides
> this one and whether the check as such is justified.

It's a debug option...

> > I suppose you could extend the check to include schedule_debug() as
> > well, maybe something like:
> 
> Do you mean to make the check cheaper?

Nah, so the patch only touched might_sleep(), the below touches
schedule().

If there were a patch that hits schedule() without going through a
might_sleep() (rare in practise I think, but entirely possible) then you
won't get a splat without something like the below on top.

> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index f66920173370..b1aaa278f1af 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3278,13 +3278,18 @@ static noinline void __schedule_bug(struct task_struct *prev)
> >  /*
> >   * Various schedule()-time debugging checks and statistics:
> >   */
> > -static inline void schedule_debug(struct task_struct *prev)
> > +static inline void schedule_debug(struct task_struct *prev, bool preempt)
> >  {
> >  #ifdef CONFIG_SCHED_STACK_END_CHECK
> >  	if (task_stack_end_corrupted(prev))
> >  		panic("corrupted stack end detected inside scheduler\n");
> >  #endif
> >  
> > +#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
> > +	if (!preempt && prev->state && prev->non_block_count)
> > +		// splat
> > +#endif
> > +
> >  	if (unlikely(in_atomic_preempt_off())) {
> >  		__schedule_bug(prev);
> >  		preempt_count_set(PREEMPT_DISABLED);
> > @@ -3391,7 +3396,7 @@ static void __sched notrace __schedule(bool preempt)
> >  	rq = cpu_rq(cpu);
> >  	prev = rq->curr;
> >  
> > -	schedule_debug(prev);
> > +	schedule_debug(prev, preempt);
> >  
> >  	if (sched_feat(HRTICK))
> >  		hrtick_clear(rq);
> 
> -- 
> Michal Hocko
> SUSE Labs