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

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Date:   Wed, 17 Jun 2020 12:29:40 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     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 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
 
> 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