linux-kernel - Re: [PATCH v2] dma-buf: add ref counting for module as exporter

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Date:	Thu, 7 May 2015 22:49:48 +0200
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Sumit Semwal <sumit.semwal@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
	linux-arm-kernel@...ts.infradead.org, rmk+kernel@....linux.org.uk,
	airlied@...ux.ie, kgene@...nel.org, thierry.reding@...il.com,
	pawel@...iak.com, m.szyprowski@...sung.com,
	mchehab@....samsung.com, linaro-kernel@...ts.linaro.org,
	robdclark@...il.com, daniel@...ll.ch
Subject: Re: [PATCH v2] dma-buf: add ref counting for module as exporter

On Thu, May 07, 2015 at 06:58:44PM +0530, Sumit Semwal wrote:
> Add reference counting on a kernel module that exports dma-buf and
> implements its operations. This prevents the module from being unloaded
> while DMABUF file is in use.
> 
> The original patch [1] was submitted by Tomasz Stanislawski, but this
> is a simpler way to do it.
> 
> v2: move owner to struct dma_buf, and use DEFINE_DMA_BUF_EXPORT_INFO
>     macro to simplify the change.
> 
> Signed-off-by: Sumit Semwal <sumit.semwal@...aro.org>
> 
> [1]: https://lkml.org/lkml/2012/8/8/163
> ---
>  drivers/dma-buf/dma-buf.c | 10 +++++++++-
>  include/linux/dma-buf.h   | 10 ++++++++--
>  2 files changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index c5a9138a6a8d..0eff4bf56ef6 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -29,6 +29,7 @@
>  #include <linux/anon_inodes.h>
>  #include <linux/export.h>
>  #include <linux/debugfs.h>
> +#include <linux/module.h>
>  #include <linux/seq_file.h>
>  #include <linux/poll.h>
>  #include <linux/reservation.h>
> @@ -64,6 +65,7 @@ static int dma_buf_release(struct inode *inode, struct file *file)
>  	BUG_ON(dmabuf->cb_shared.active || dmabuf->cb_excl.active);
>  
>  	dmabuf->ops->release(dmabuf);
> +	module_put(dmabuf->owner);

The module can now go away.

>  
>  	mutex_lock(&db_list.lock);
>  	list_del(&dmabuf->list_node);

But you reference it here :(

Please drop the reference at the last possible moment.

thanks,

greg k-h
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