Re: [PATCH 2/2] drm/scheduler: Remove obsolete spinlock.

From: "Christian König" <christian.koenig-5C7GfCeVMHo@public.gmane.org>
To: Lucas Stach <l.stach-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org>,
	Andrey Grodzovsky
	<Andrey.Grodzovsky-5C7GfCeVMHo@public.gmane.org>,
	"Zhou,
	David(ChunMing)" <David1.Zhou-5C7GfCeVMHo@public.gmane.org>,
	"amd-gfx-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org"
	<amd-gfx-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org>,
	"dri-devel-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org"
	<dri-devel-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org>
Subject: Re: [PATCH 2/2] drm/scheduler: Remove obsolete spinlock.
Date: Wed, 16 May 2018 15:10:39 +0200	[thread overview]
Message-ID: <96b4e712-5e80-0053-301e-3f89940be8f0@amd.com> (raw)
In-Reply-To: <1526475600.10230.3.camel-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org>

Am 16.05.2018 um 15:00 schrieb Lucas Stach:
> Am Mittwoch, den 16.05.2018, 14:32 +0200 schrieb Christian König:
>> Am 16.05.2018 um 14:28 schrieb Lucas Stach:
>>> Am Mittwoch, den 16.05.2018, 14:08 +0200 schrieb Christian König:
>>>> Yes, exactly.
>>>>
>>>> For normal user space command submission we should have tons of
>>>> locks
>>>> guaranteeing that (e.g. just the VM lock should do).
>>>>
>>>> For kernel moves we have the mutex for the GTT windows which
>>>> protects
>>>> it.
>>>>
>>>> The could be problems with the UVD/VCE queues to cleanup the
>>>> handles
>>>> when an application crashes.
>>> FWIW, etnaviv is currently completely unlocked in this path, but I
>>> believe that this isn't an issue as the sched fence seq numbers are
>>> per
>>> entity. So to actually end up with reversed seqnos one context has
>>> to
>>> preempt itself to do another submit, while the current one hasn't
>>> returned from kernel space, which I believe is a fairly theoretical
>>> issue. Is my understanding correct?
>> Yes. The problem is with the right timing this can be used to access
>> freed up memory.
>>
>> If you then manage to place a page table in that freed up memory
>> taking
>> over the system is just a typing exercise.
> Thanks. I believe we don't have this problem in etnaviv, as memory
> referencing is tied to the job and will only be unreferenced on
> free_job, but I'll re-check this when I've got some time.

Well that depends on how you use the sequence numbers.

If you use them for job completion check somewhere then you certainly 
have a problem if job A gets the 1 and B the 2, but B completes before A.

At bare minimum that's still a bug we need to fix because it confuses 
functions like dma_fence_is_later() and dma_fence_later().

Christian.

>
> Regards,
> Lucas

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