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Date:   Fri, 23 Nov 2018 09:46:09 +0100
From:   Daniel Vetter <daniel@ffwll.ch>
To:     "Koenig, Christian" <Christian.Koenig@amd.com>
Cc:     Daniel Vetter <daniel.vetter@ffwll.ch>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        David Rientjes <rientjes@google.com>,
        =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
        Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [PATCH 2/3] mm, notifier: Catch sleeping/blocking for !blockable
Message-ID: <20181123084609.GH4266@phenom.ffwll.local>
Mail-Followup-To: "Koenig, Christian" <Christian.Koenig@amd.com>,
        LKML <linux-kernel@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        David Rientjes <rientjes@google.com>,
        =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
        Daniel Vetter <daniel.vetter@intel.com>
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On Thu, Nov 22, 2018 at 06:55:17PM +0000, Koenig, Christian wrote:
> Am 22.11.18 um 17:51 schrieb Daniel Vetter:
> > We need to make sure implementations don't cheat and don't have a
> > possible schedule/blocking point deeply burried where review can't
> > catch it.
> >
> > I'm not sure whether this is the best way to make sure all the
> > might_sleep() callsites trigger, and it's a bit ugly in the code flow.
> > But it gets the job done.
> >
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: David Rientjes <rientjes@google.com>
> > Cc: "Christian K?nig" <christian.koenig@amd.com>
> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > Cc: "J?r?me Glisse" <jglisse@redhat.com>
> > Cc: linux-mm@kvack.org
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > ---
> >   mm/mmu_notifier.c | 8 +++++++-
> >   1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> > index 59e102589a25..4d282cfb296e 100644
> > --- a/mm/mmu_notifier.c
> > +++ b/mm/mmu_notifier.c
> > @@ -185,7 +185,13 @@ int __mmu_notifier_invalidate_range_start(struct mm_struct *mm,
> >   	id = srcu_read_lock(&srcu);
> >   	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
> >   		if (mn->ops->invalidate_range_start) {
> > -			int _ret = mn->ops->invalidate_range_start(mn, mm, start, end, blockable);
> > +			int _ret;
> > +
> > +			if (IS_ENABLED(CONFIG_DEBUG_ATOMIC_SLEEP) && !blockable)
> > +				preempt_disable();
> > +			_ret = mn->ops->invalidate_range_start(mn, mm, start, end, blockable);
> > +			if (IS_ENABLED(CONFIG_DEBUG_ATOMIC_SLEEP) && !blockable)
> > +				preempt_enable();
> 
> Just for the sake of better documenting this how about adding this to 
> include/linux/kernel.h right next to might_sleep():
> 
> #define disallow_sleeping_if(cond)??? for((cond) ? preempt_disable() : 
> (void)0; (cond); preempt_disable())
> 
> (Just from the back of my head, might contain peanuts and/or hints of 
> errors).

I think these magic for blocks aren't used in the kernel. goto breaks
them, and we use goto a lot. I think a disallow/allow_sleep() pair with
the conditional preept_disable/enable() calls would be nice though. I can
do that if the overall idea sticks.
-Daniel

> 
> Christian.
> 
> >   			if (_ret) {
> >   				pr_info("%pS callback failed with %d in %sblockable context.\n",
> >   						mn->ops->invalidate_range_start, _ret,
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch