Date: Thu, 15 May 2008 07:30:07 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-arch@freebsd.org Cc: Kostik Belousov <kostikbel@gmail.com> Subject: Re: Per-open file private data for the cdevs Message-ID: <200805150730.08418.jhb@freebsd.org> In-Reply-To: <20080514093223.GN18958@deviant.kiev.zoral.com.ua> References: <20080504171002.GN18958@deviant.kiev.zoral.com.ua> <20080513093734.GF18958@deviant.kiev.zoral.com.ua> <20080514093223.GN18958@deviant.kiev.zoral.com.ua>
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On Wednesday 14 May 2008 05:32:23 am Kostik Belousov wrote: > On Tue, May 13, 2008 at 12:37:34PM +0300, Kostik Belousov wrote: > > On Mon, May 12, 2008 at 10:03:14AM -1000, Jeff Roberson wrote: > > > > > > On Mon, 12 May 2008, Kostik Belousov wrote: > > > > > > >On Sun, May 11, 2008 at 03:40:14PM -1000, Jeff Roberson wrote: > > > >> > > > >>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. > > > >I use the f_cdevpriv != NULL as an indicator for the necessity to enter > > > >the cdevpriv code, in particular, locking the cdevpriv_mtx, that would > > > >otherwise needed to be entered at each last close. I think that one > > > >pointer for the struct file is not too big cost, do you agree ? > > > > > > No, it's not a big cost, however if it is possible to avoid that is best. > > > > > > Can you not check the type before checking f_cdevpriv? Should we not only > > > be checking cdevpriv in contexts where we know that it is not a vnode? > > > > I am sorry, my english may be not enough, so I may interpret your > > proposal mistakenly. I read it as a suggestion to check the file type > > before accessing the f_cdevpriv. > > > > The problem with the f_cdevpriv exists only at the _fdrop(). There, we > > have a file of f_type == DTYPE_VNODE both for devfs and normal files. > > I cannot check the f_vnode since the vnode may be reclaimed. The only > > differentiator is the f_ops, that is devfs_ops_f for devfs file, and > > vnops for the normal file during the file lifetime. Unfortunately, > > f_ops is reset to the badfileops by vn_closefile before the _fdrop() is > > getting called. > > > > Reserving the flag in the f_flag looks not good due to interaction with > > the userspace. > > > > I do not want the callback to be called before the d_close() driver method > > gets a chance to clean the data. > > So, I cannot implement overlay of the cdevpriv over the seqaccess data > without some additional flag. On the other hand, use of 2 bytes for the > f_type is overkill when the range of the types is [1,8]. I divided it > to two one-byte fields, and new field is used for filetype-specific > flags. > > http://people.freebsd.org/~kib/misc/fdpriv.4.patch > shall give the same size of the struct file while not causing unconditional > acquire of the cdevpriv_mtx on the last file close. > > Jeff, do you agree with the proposed vivisection of the f_type ? I will > ask Peter to retest the patch then. I would rather add the new pointer to struct file and avoid breaking the ABI of fstat. That is, I would like this patch to be MFCable, but you can't MFC this w/o breaking the ABI of struct file since fstat(1) will be reading the full short to get the DTYPE. -- John Baldwin
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