Date: Mon, 2 Jul 2012 18:17:48 -0400 From: Alexander Kabaev <kabaev@gmail.com> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Attilio Rao <attilio@freebsd.org>, "C. P. Ghost" <cpghost@cordula.ws>, Russell, Christoph Hellwig <hch@infradead.org>, Cattelan <cattelan@thebarn.com>, freebsd-current@freebsd.org, FreeBSD FS <freebsd-fs@freebsd.org> Subject: Re: MPSAFE VFS -- List of upcoming actions Message-ID: <20120702181748.6bb126f6@kan.dyndns.org> In-Reply-To: <20120702201206.GV2337@deviant.kiev.zoral.com.ua> References: <CAJ-FndAJtFx_OhqzDvBSLQ5pEaX730oF8Tbyk%2BkYbz9y1KaXXA@mail.gmail.com> <CADGWnjXPtF1g1YXWEie3VAhamjj3D_MQ89Ep4zh3_6g8tGHzAA@mail.gmail.com> <CAJ-FndDRvZdzF00hO6TWJMASDmpgK4mkF3GFsacF3KBSB00YWw@mail.gmail.com> <20120702061219.GA16671@infradead.org> <20120702150339.GA7226@kan.dyndns.org> <20120702201206.GV2337@deviant.kiev.zoral.com.ua>
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--Sig_/tC9gBXvdben2oNR.d08Mkfp Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable On Mon, 2 Jul 2012 23:12:06 +0300 Konstantin Belousov <kostikbel@gmail.com> wrote: > On Mon, Jul 02, 2012 at 11:03:40AM -0400, Alexander Kabaev wrote: > > On Mon, Jul 02, 2012 at 02:12:20AM -0400, Christoph Hellwig wrote: > > > On Sun, Jul 01, 2012 at 03:52:05PM +0200, Attilio Rao wrote: > > > > anything by SoC involved people about NTFS and certainly I > > > > don't see a plan to get XFS locked. > > >=20 > > > Stupid question, but what amount of locking does XFS in FreeBSD > > > still need? I'm one of the maintainer of XFS on Linux, and while > > > I know FreeBSD imported a really old version compared to the > > > current one the codebases on IRIX and later Linux never relied on > > > any global Giant-style locking. So if there is anything to fix > > > it would be the in the small bits of FreeBSD-specific code. > > >=20 > >=20 > > When I stopped being interested in XFS, I left is marked as > > non-MPSAFE entirely because of the lack of proper testing and > > because VFS locking was still evolving, there was no officially > > proper way of locking the FS and no other FS in the tree was > > MPSAFE. At that time the only problematic area was around inode > > instantiation, but sereval other lockingi changes have made it into > > the tree since then, namely ones that deal with insmntque and also > > VOP_LOOKUP changes. To mark XFS MPSAFE, one needs to simply audit > > the code and make sure it still makse sense for today's VFS, which > > is not a huge amount of work. One step further woule be to take > > most of the XFS from under the exclusive vnode locking to improve > > the performance. >=20 > If filesystem uses some global internal locks, that locks usually are > placed after the vnode locks in global lock order, because VOP methods > call into fs with vnode locked. Then, VOP_LOOKUP() usual sequence of > events, when method is called with lookup directory locked, causes > LOR. It appears because you lock global lock upon entry into > VOP_LOOKUP(), and then need to lock the returned vnode. > Dropping global lock inside VOP_LOOKUP() usually exposes races which > were the reason to introduce the global lock. Having filesystem > non-MPSAFE makes the LOR go away without the need to drop global lock. >=20 > Example of FreeBSD native filesystem which suffered from this issue > and required quite non-trivial handling is devfs. Devfs uses per-mount > global lock. See devfs_allocv() and devfs_allocv_drop_refs() for > the gory details. All very informative, though misplaced in case of XFS. XFS does not use global lock internally, it is quite well locked inside and exclusive _VNODE_ lock we take by default in all methods is usually unnecessary as all it does it prevents other locks in XFS proper from having any useful function. --=20 Alexander Kabaev --Sig_/tC9gBXvdben2oNR.d08Mkfp Content-Type: application/pgp-signature; name=signature.asc Content-Disposition: attachment; filename=signature.asc -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.19 (FreeBSD) iD8DBQFP8h4VQ6z1jMm+XZYRAhbnAJwJ9FKxAHEhZQS3re4WJZZA5Jr4+wCcCXv8 AA5y7bwN5mHu/RbP0pxXh1M= =u2RK -----END PGP SIGNATURE----- --Sig_/tC9gBXvdben2oNR.d08Mkfp--
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