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

[Daniel Vetter]

On Tue, Oct 19, 2021 at 05:37:27PM +0200, Christian König wrote:
> 
> 
> 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.

I think if done right the object debug stuff should be able to give you a
backtrace. Which might be useful if you have a dma-buf heaps design where
you really have no clue why a buffer was allocated/attached without some
hints.

> > 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));".

Yeah this patch here alone isn't really that useful I think. Maybe we
could add the dmabuf->exp_name or so to that warning, but otherwise the
info printed here isn't all that useful for debugging. Grabbing a
backtrace of the allocator or attacher otoh should fairly immedialy point
at the buggy code.
-Daniel

> 
> 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
> > > 
> 

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