Date: Wed, 14 May 2008 12:32:23 +0300 From: Kostik Belousov <kostikbel@gmail.com> To: Jeff Roberson <jroberson@jroberson.net> Cc: arch@freebsd.org Subject: Re: Per-open file private data for the cdevs Message-ID: <20080514093223.GN18958@deviant.kiev.zoral.com.ua> In-Reply-To: <20080513093734.GF18958@deviant.kiev.zoral.com.ua> References: <20080504171002.GN18958@deviant.kiev.zoral.com.ua> <20080510214812.Y954@desktop> <20080511115030.GV18958@deviant.kiev.zoral.com.ua> <20080511115815.GW18958@deviant.kiev.zoral.com.ua> <20080511153926.T954@desktop> <20080512101607.GC18958@deviant.kiev.zoral.com.ua> <20080512100202.W954@desktop> <20080513093734.GF18958@deviant.kiev.zoral.com.ua>
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--5glgGqCv79Y86mRH Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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: > >=20 > > On Mon, 12 May 2008, Kostik Belousov wrote: > >=20 > > >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 som= e=20 > > >>>>>>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 func= tions > > >>>>>>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. d= tr > > >>>>>>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 sti= ll > > >>>>>>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= =20 > > >>>>>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=20 > > >>>>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 !=3D NULL as an indicator for the necessity to en= ter > > >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 ? > >=20 > > No, it's not a big cost, however if it is possible to avoid that is bes= t. > >=20 > > Can you not check the type before checking f_cdevpriv? Should we not o= nly=20 > > be checking cdevpriv in contexts where we know that it is not a vnode? >=20 > 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. >=20 > The problem with the f_cdevpriv exists only at the _fdrop(). There, we > have a file of f_type =3D=3D 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. >=20 > Reserving the flag in the f_flag looks not good due to interaction with > the userspace. >=20 > 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=20 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. --5glgGqCv79Y86mRH Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (FreeBSD) iEYEARECAAYFAkgqsaYACgkQC3+MBN1Mb4hPJQCgnwivOryzU6WwVdASNvY0MNXb UT8AoOL6Cg7J3i0Ro3xNTMYtszAVFpvd =AbHK -----END PGP SIGNATURE----- --5glgGqCv79Y86mRH--
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