linux-kernel - Re: [Linaro-mm-sig] [PATCH 04/18] dma-fence: prime lockdep annotations

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Message-ID: <20200618144236.GR20149@phenom.ffwll.local>
Date:   Thu, 18 Jun 2020 16:42:36 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Daniel Vetter <daniel@...ll.ch>,
        Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Thomas Hellstrom <thomas.hellstrom@...el.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        Christian König <christian.koenig@....com>,
        Mika Kuoppala <mika.kuoppala@...el.com>
Subject: Re: [Linaro-mm-sig] [PATCH 04/18] dma-fence: prime lockdep
 annotations

On Wed, Jun 17, 2020 at 12:29:40PM -0300, Jason Gunthorpe wrote:
> On Wed, Jun 17, 2020 at 09:57:54AM +0200, Daniel Vetter wrote:
> 
> > > At the very least I think there should be some big warning that
> > > dma_fence in notifiers should be avoided.
> > 
> > Yeah I'm working on documentation, and also the notifiers here
> > hopefully make it clear it's massive pain. I think we could even make
> > a hard rule that dma_fence in mmu notifier outside of drivers/gpu is a
> > bug/misfeature.
> 
> Yep!
>
> > Might be a good idea to add a MAINTAINERS entry with a K: regex
> > pattern, so that you can catch such modifiers. We do already have such
> > a pattern for dma-fence, to catch abuse. So if you want I could type
> > up a documentation patch for this, get your and others acks and the
> > dri-devel folks would enforce that the dma_fence_wait madness doesn't
> > leak beyond drivers/gpu
> 
> It seems like the best thing

Just thought about where to best put this, and I think including it as
another paragraph in the next round of this series makes the most sense.
You'll get cc'ed for acking when that happens - might take a while since
there's a lot of details here all over to sort out.
-Daniel

>  
> > Oded has agreed to remove the dma-fence usage, since they really don't
> > need it (and all the baggage that comes with it), plain old completion
> > is enough for their use. This use is also why I added the regex to
> > MAINTAINERS, so that in the future we can catch people who try to use
> > dma_fence because it looks cute and useful, and are completely
> > oblivious to all the pain and headaches involved.
> 
> This is good!
> 
> Thanks,
> Jason 

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