Per-open file private data for the cdevs

[Jeff Roberson]

On Sun, 11 May 2008, Kostik Belousov wrote:

> On Sun, May 11, 2008 at 02:50:30PM +0300, Kostik Belousov wrote:
>> On Sat, May 10, 2008 at 09:53:12PM -1000, Jeff Roberson wrote:
>>> On Sun, 4 May 2008, Kostik Belousov wrote:
>>>
>>>> Since the review for the clone-at-open patch (fdclone) posted some time ago
>>>> mostly says that it would be better to implement per-file private data
>>>> instead, I produced the patch along this line,
>>>>
>>>> The patch does not change the cdevsw ABI, instead, three new functions
>>>> nt	devfs_get_cdevpriv(void **datap);
>>>> int	devfs_set_cdevpriv(void *priv, cdevpriv_dtr_t dtr);
>>>> void	devfs_clear_cdevpriv(void);
>>>> are provided for manipulation of the per-file private data.
>>>>
>>>> devfs_set_cdevpriv assigns the priv as private data for the file descriptor
>>>> which is used to initiate currently performed driver operation. dtr
>>>> is the function that will be called when either the last refernce to
>>>> the file goes away or devfs_clear_cdevpriv is called.
>>>>
>>>> devfs_get_cdevpriv is the obvious accessor.
>>>>
>>>> devfs_clear_cdevpriv allows to clear the private data for the still
>>>> open file.
>>>>
>>>> The synchronization of the cdev data and file private data is left
>>>> to the driver code, I did not found any generic helper mechanism that
>>>> could be useful there.
>>>>
>>>> Patch:
>>>> http://people.freebsd.org/~kib/misc/fdpriv.1.patch
>>>>
>>>> Dumb driver that shows the basic usage of the proposed KPI:
>>>> http://people.freebsd.org/~kib/misc/fpclone.c
>>>>
>>>> Previous version of the patch was tested by Peter Holm.
>>>>
>>>
>>> Hi Kostik,
>>>
>>> Are these per-instances structures intended to be used by anything other
>>> than devices?  If not can we make them a union with the DTYPE_VNODE
>>> fields to save space?
>>>
>>> Thanks,
>>> Jeff
>>
>> The current version of the patch is at
>> http://people.freebsd.org/~kib/misc/fdpriv.3.patch
>>
>> Per insistence of John Baldwin and request of Eric Anholt, the destructors
>> are called now when either file is last closed, or the device is destroyed.
>> This versions adds only one pointer to the struct file.
>>
>> Jeff, would you, please, explicitely specify what field you propose to
>> union with the f_cdevpriv ?

f_nextoff and f_seqcount are only used if vn_read() and vn_write() are 
used.  They do not apply to any other descriptors.

>
> Of course, I forgot to answer the question. Yes, the KPI is supposed to
> be used by the drivers only. Please, note that device open files have
> DTYPE_VNODE.
>