Re: [Linaro-mm-sig] [PATCH] dma-buf: add attachments empty check for dma_buf_release

[Christian König]

Am 19.10.21 um 14:41 schrieb Daniel Vetter:
> On Tue, Oct 19, 2021 at 08:23:45PM +0800, guangming.cao@xxxxxxxxxxxx wrote:
> > From: Guangming Cao <Guangming.Cao@xxxxxxxxxxxx>
> >
> > Since there is no mandatory inspection for attachments in dma_buf_release.
> > There will be a case that dma_buf already released but attachment is still
> > in use, which can points to the dmabuf, and it maybe cause
> > some unexpected issues.
> >
> > With IOMMU, when this cases occurs, there will have IOMMU address
> > translation fault(s) followed by this warning,
> > I think it's useful for dma devices to debug issue.
> >
> > Signed-off-by: Guangming Cao <Guangming.Cao@xxxxxxxxxxxx>
> This feels a lot like hand-rolling kobject debugging. If you want to do
> this then I think adding kobject debug support to
> dma_buf/dma_buf_attachment would be better than hand-rolling something
> bespoke here.

Well I would call that overkill.

> Also on the patch itself: You don't need the trylock. For correctly
> working code non one else can get at the dma-buf, so no locking needed to
> iterate through the attachment list. For incorrect code the kernel will be
> on fire pretty soon anyway, trying to do locking won't help :-) And
> without the trylock we can catch more bugs (e.g. if you also forgot to
> unlock and not just forgot to detach).

You also don't need the WARN(!list_empty...) because a few line below we 
already have a "WARN_ON(!list_empty(&dmabuf->attachments));".

Christian.

> -Daniel
>
> > ---
> >   drivers/dma-buf/dma-buf.c | 23 +++++++++++++++++++++++
> >   1 file changed, 23 insertions(+)
> >
> > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > index 511fe0d217a0..672404857d6a 100644
> > --- a/drivers/dma-buf/dma-buf.c
> > +++ b/drivers/dma-buf/dma-buf.c
> > @@ -74,6 +74,29 @@ static void dma_buf_release(struct dentry *dentry)
> >   	 */
> >   	BUG_ON(dmabuf->cb_shared.active || dmabuf->cb_excl.active);
> >   
> > +	/* attachment check */
> > +	if (dma_resv_trylock(dmabuf->resv) && WARN(!list_empty(&dmabuf->attachments),
> > +	    "%s err, inode:%08lu size:%08zu name:%s exp_name:%s flags:0x%08x mode:0x%08x, %s\n",
> > +	    __func__, file_inode(dmabuf->file)->i_ino, dmabuf->size,
> > +	    dmabuf->name, dmabuf->exp_name,
> > +	    dmabuf->file->f_flags, dmabuf->file->f_mode,
> > +	    "Release dmabuf before detach all attachments, dump attach:\n")) {
> > +		int attach_cnt = 0;
> > +		dma_addr_t dma_addr;
> > +		struct dma_buf_attachment *attach_obj;
> > +		/* dump all attachment info */
> > +		list_for_each_entry(attach_obj, &dmabuf->attachments, node) {
> > +			dma_addr = (dma_addr_t)0;
> > +			if (attach_obj->sgt)
> > +				dma_addr = sg_dma_address(attach_obj->sgt->sgl);
> > +			pr_err("attach[%d]: dev:%s dma_addr:0x%-12lx\n",
> > +			       attach_cnt, dev_name(attach_obj->dev), dma_addr);
> > +			attach_cnt++;
> > +		}
> > +		pr_err("Total %d devices attached\n\n", attach_cnt);
> > +		dma_resv_unlock(dmabuf->resv);
> > +	}
> > +
> >   	dmabuf->ops->release(dmabuf);
> >   
> >   	if (dmabuf->resv == (struct dma_resv *)&dmabuf[1])
> > --
> > 2.17.1