From: "Christian König" <christian.koenig@amd.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Leon Romanovsky <leon@kernel.org>,
kvm@vger.kernel.org, linux-rdma@vger.kernel.org,
Daniel Vetter <daniel.vetter@ffwll.ch>,
Oded Gabbay <ogabbay@kernel.org>,
Cornelia Huck <cohuck@redhat.com>,
dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org,
Alex Williamson <alex.williamson@redhat.com>,
Maor Gottlieb <maorg@nvidia.com>,
Sumit Semwal <sumit.semwal@linaro.org>,
linux-media@vger.kernel.org
Subject: Re: [PATCH 0/4] Allow MMIO regions to be exported through dma-buf
Date: Thu, 18 Aug 2022 15:37:01 +0200 [thread overview]
Message-ID: <23cb08e4-6de8-8ff4-b569-c93533bf0e19@amd.com> (raw)
In-Reply-To: <Yv47lkz7FG8vhqA5@nvidia.com>
Am 18.08.22 um 15:16 schrieb Jason Gunthorpe:
> On Thu, Aug 18, 2022 at 02:58:10PM +0200, Christian König wrote:
>
>>>> The only thing I'm not 100% convinced of is dma_buf_try_get(), I've seen
>>>> this incorrectly used so many times that I can't count them any more.
>>>>
>>>> Would that be somehow avoidable? Or could you at least explain the use case
>>>> a bit better.
>>> I didn't see a way, maybe you know of one
>> For GEM objects we usually don't use the reference count of the DMA-buf, but
>> rather that of the GEM object for this. But that's not an ideal solution
>> either.
> You can't really ignore the dmabuf refcount. At some point you have to
> deal with the dmabuf being asynchronously released by userspace.
Yeah, but in this case the dma-buf is just a reference to the
real/private object which holds the backing store.
When the dma-buf is released you drop the real object reference and from
your driver internals you only try_get only the real object.
The advantage is that only your driver can use the try_get function and
not some importing driver which doesn't know about the internals of the
exporter.
We just had to many cases where developers weren't sure if a pointer is
still valid and by using try_get it just "magically" got working (well I
have to admit it made the crashing less likely....).
>>> down_write(&vdev->memory_lock);
>>> list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) {
>>> if (!dma_buf_try_get(priv->dmabuf))
>>> continue;
>> What would happen if you don't skip destroyed dma-bufs here? In other words
>> why do you maintain that list in the first place?
> The list is to keep track of the dmabufs that were created, it is not
> optional.
>
> The only question is what do you do about invoking
> dma_buf_move_notify() on a dmabuf that is already undergoing
> destruction.
Ah, yes. Really good point.
>
> For instance undergoing destruction means the dmabuf core has already
> done this:
>
> mutex_lock(&db_list.lock);
> list_del(&dmabuf->list_node);
> mutex_unlock(&db_list.lock);
> dma_buf_stats_teardown(dmabuf);
>
> So it seems non-ideal to continue to use it.
>
> However, dma_buf_move_notify() specifically has no issue with that level of
> destruction since it only does
>
> list_for_each_entry(attach, &dmabuf->attachments, node)
>
> And attachments must be empty if the file refcount is zero.
>
> So we could delete the try_buf and just rely on move being safe on
> partially destroyed dma_buf's as part of the API design.
I think that might be the more defensive approach. A comment on the
dma_buf_move_notify() function should probably be a good idea.
Thanks,
Christian.
>
> Jason
next prev parent reply other threads:[~2022-08-18 13:38 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-17 16:11 [PATCH 0/4] Allow MMIO regions to be exported through dma-buf Jason Gunthorpe
2022-08-17 16:11 ` [PATCH 1/4] dma-buf: Add dma_buf_try_get() Jason Gunthorpe
2022-08-17 16:11 ` [PATCH 2/4] vfio: Add vfio_device_get() Jason Gunthorpe
2022-08-17 16:11 ` [PATCH 3/4] vfio_pci: Do not open code pci_try_reset_function() Jason Gunthorpe
2022-08-17 16:11 ` [PATCH 4/4] vfio/pci: Allow MMIO regions to be exported through dma-buf Jason Gunthorpe
2022-08-21 13:51 ` Fwd: " Oded Gabbay
2022-08-26 18:10 ` Jason Gunthorpe
2022-08-29 5:04 ` Yan Zhao
2022-08-29 12:26 ` Jason Gunthorpe
2022-08-18 11:07 ` [PATCH 0/4] " Christian König
2022-08-18 12:03 ` Jason Gunthorpe
2022-08-18 12:58 ` Christian König
2022-08-18 13:16 ` Jason Gunthorpe
2022-08-18 13:37 ` Christian König [this message]
2022-08-19 13:11 ` Jason Gunthorpe
2022-08-19 13:33 ` Christian König
2022-08-19 13:39 ` Jason Gunthorpe
2022-08-19 13:47 ` Christian König
2022-08-18 12:05 ` Jason Gunthorpe
2022-08-22 21:58 ` Alex Williamson
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