linux-kernel - Re: [PATCH RFC RESEND] dma-buf/fs Add get_[file|dma_buf]_unless

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Date:	Mon, 11 Nov 2013 15:17:04 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Thomas Hellstrom <thellstrom@...are.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC RESEND] dma-buf/fs Add
 get_[file|dma_buf]_unless_doomed

On Mon, Nov 11, 2013 at 12:57:47AM -0800, Thomas Hellstrom wrote:
> Resending since it appears this RFC never got to the dri-devel lkml lists.
> 
> In this context, a "doomed" object is an object whose refcount has reached
> zero, but that has not yet been freed.
> 
> To avoid mutual refcounting vmwgfx need to have a non-refcounted pointer to
> a dma-buf in a lookup structure. The pointer is removed in the dma-buf
> destructor. To allow lookup-structure private locks we need
> get_dma_buf_unless_doomed(). This common refcounting scenario is described
> with examples in detail in the kref documentaion.
> The solution with local locks is under kref_get_unless_zero().
> See also kobject_get_unless_zero() and its commit message.
> Since dma-bufs are using the attached file for refcounting,
> get_dma_buf_unless_doomed maps directly to a get_file_unless_doomed.

NAK for struct file.  This kind of stuff is for implementing primitives,
not as a public API.  BTW, as for dmabuf...  dma_buf_fd() calling conventions
are seriously misguided - we are trying to transfer the reference we hold
into something (in this case - descriptor table), so the failure exit
should be dropping the reference, not leaving that to caller.
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