| Message ID | 20241216095429.210792-1-wguay@fb.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | cover-letter: Allow MMIO regions to be exported through dmabuf | expand |
Am 17.12.24 um 12:06 schrieb Wei Lin Guay: > [SNIP] >> Please send that out once more with me on explicit CC. > I will resend the patch series. I was experiencing issues with my email client, which inadvertently split the series into two separate emails. Alternatively I can also copy the code from the list archive and explain why that doesn't work: +void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked) +{ + struct vfio_pci_dma_buf *priv; + struct vfio_pci_dma_buf *tmp; + + lockdep_assert_held_write(&vdev->memory_lock); This makes sure that the caller is holding vdev->memory_lock. + + list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) { + if (!dma_buf_try_get(priv->dmabuf)) This here only works because vfio_pci_dma_buf_release() also grabs vdev->memory_lock and so we are protected against concurrent releases. + continue; + if (priv->revoked != revoked) { + dma_resv_lock(priv->dmabuf->resv, NULL); + priv->revoked = revoked; + dma_buf_move_notify(priv->dmabuf); + dma_resv_unlock(priv->dmabuf->resv); + } + dma_buf_put(priv->dmabuf); The problem is now that this here might drop the last reference which in turn calls vfio_pci_dma_buf_release() which also tries to grab vdev->memory_lock and so results in a deadlock. + } +} This pattern was suggested multiple times and I had to rejected it every time because the whole idea is just fundamentally broken. It's really astonishing how people always come up with the same broken pattern. Regards, Christian. > >> Apart from that I have to reject the adding of dma_buf_try_get(), that is clearly not something we should do. > > Understood. It appears that Vivek has confirmed that his v2 has resolved the issue. I will follow up with him to determine if he plans to resume his patch, and if so, I will apply my last patch on top of his updated patch series > > Thanks, > Wei Lin >> Thanks, >> Christian. >
Hi Christian, > Subject: Re: [PATCH 0/4] cover-letter: Allow MMIO regions to be exported > through dmabuf > > >> I will resend the patch series. I was experiencing issues with my email >> client, which inadvertently split the series into two separate emails. > > > Alternatively I can also copy the code from the list archive and explain why > that doesn't work: > > +void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool > +revoked) { > + struct vfio_pci_dma_buf *priv; > + struct vfio_pci_dma_buf *tmp; > + > + lockdep_assert_held_write(&vdev->memory_lock); > > This makes sure that the caller is holding vdev->memory_lock. > > + > + list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) { > + if (!dma_buf_try_get(priv->dmabuf)) > > This here only works because vfio_pci_dma_buf_release() also grabs vdev- > >memory_lock and so we are protected against concurrent releases. > > + continue; > + if (priv->revoked != revoked) { > + dma_resv_lock(priv->dmabuf->resv, NULL); > + priv->revoked = revoked; > + dma_buf_move_notify(priv->dmabuf); > + dma_resv_unlock(priv->dmabuf->resv); > + } > + dma_buf_put(priv->dmabuf); > > The problem is now that this here might drop the last reference which in turn > calls vfio_pci_dma_buf_release() which also tries to grab vdev- > >memory_lock and so results in a deadlock. AFAICS, vfio_pci_dma_buf_release() would not be called synchronously after the last reference is dropped by dma_buf_put(). This is because fput(), which is called by dma_buf_put() triggers f_op->release() asynchronously; therefore, a deadlock is unlikely to occur in this scenario, unless I am overlooking something. Thanks, Vivek > > + } > +} > > This pattern was suggested multiple times and I had to rejected it every time > because the whole idea is just fundamentally broken. > > It's really astonishing how people always come up with the same broken > pattern. > > Regards, > Christian. > > > > > > > > Apart from that I have to reject the adding of > dma_buf_try_get(), that is clearly not something we should do. > > > > Understood. It appears that Vivek has confirmed that his v2 has > resolved the issue. I will follow up with him to determine if he plans to > resume his patch, and if so, I will apply my last patch on top of his updated > patch series > > Thanks, > Wei Lin > > > Thanks, > Christian. > > > >