Date: Wed, 27 Oct 2010 18:34:02 +0300 From: Kostik Belousov <kostikbel@gmail.com> To: Benjamin Kaduk <kaduk@MIT.EDU> Cc: freebsd-hackers@freebsd.org, Sergey Kandaurov <pluknet@gmail.com>, Lars Hartmann <lars@chaotika.org> Subject: Re: [PATCH] Fix 'implicit declaration' warning and update vgone(9) Message-ID: <20101027153402.GS2392@deviant.kiev.zoral.com.ua> In-Reply-To: <alpine.GSO.1.10.1010271043550.19200@multics.mit.edu> References: <1288160610.4280.18.camel@apollon> <AANLkTimCN%2BY4OsJRNvdmXidLu8XPHz5RydAhe0phvgnH@mail.gmail.com> <201010270912.47076.jhb@freebsd.org> <alpine.GSO.1.10.1010271043550.19200@multics.mit.edu>
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--37ZONDoqvyhZS3Cx Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Oct 27, 2010 at 10:59:56AM -0400, Benjamin Kaduk wrote: > On Wed, 27 Oct 2010, John Baldwin wrote: >=20 > >On Wednesday, October 27, 2010 7:33:13 am Sergey Kandaurov wrote: > >>On 27 October 2010 10:23, Lars Hartmann <lars@chaotika.org> wrote: > >>>The vgonel function isnt declarated in any header, the vgonel prototype > >>>in vgone(9) isnt correct - found by Ben Kaduk <kaduk@mit.edu> > >> > >>Hi. > >> > >>I'm afraid it's just an overlooked man page after many VFS changes in 5= .x. > >>As vgonel() is a static (i.e. private and not visible from outside)=20 > >>function > >>IMO it should be removed from vgone(9) man page. > > > >Agreed. It certainly should not be added to vnode.h. I'm curious how t= he > >reporter is getting a warning since there is a static prototype for=20 > >vgonel() > >in vfs_subr.c. >=20 > It's for a third-party kernel module, for the OpenAFS filesystem (which= =20 > has been unloved for FreeBSD since the 4.X days). The AFS code is=20 > currently using unlocked accesses to v_usecount (which, unsurprisingly,= =20 > led to a race condition that caused an invariant check to fail), and I wa= s=20 > going through and adding the interlock around them. At the place that I= =20 > suspect to be the main cause of this race [1], the usecount was checked t= o=20 > be nonpositive along with a couple other conditions, and a little later= =20 > vgone() was called. Holding the interlock across both of these calls (an= d=20 > therefore using vgonel()) seems to have closed the race condition I was= =20 > seeing. (Other checks of v_usecount were changed to grab the interlock,= =20 > but drop it before doing anything else.) > However, looking at the commit message for vfs_subr.c:1.631, I guess this= =20 > is not the locking strategy I'm supposed to be using? >=20 > I saw a warning of implicit declaration when compiling the kernel module,= =20 > but the kernel linker was happy to load it. I forget whether it matters= =20 > that vgonel is only declared static at its declaration but not its=20 > definition. >=20 > -Ben Kaduk >=20 >=20 > [1] The old (racy) function is osi_TryEvictVCache, here: > http://git.openafs.org/?p=3Dopenafs.git;a=3Dblob;f=3Dsrc/afs/FBSD/osi_vca= che.c;h=3Dc2060c74f0155a610d2ea94f3c7f508e8ca4373a;hb=3DHEAD The function looks very strange for much more serious reasons. Why do you try to manage the vnode revocation in the filesystem module at all ? VFS will (assumedly in a right way) revoke and destroy non-referenced vnodes when needed. Anyway, you need to hold vnode lock before checking for the vnode refcounte= r. See the vfs_subr.c:vlrureclaim() for the correct dance with vholdl()/vn_loc= k() sequence that does the revocation I mentioned in the race-free way. --37ZONDoqvyhZS3Cx Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (FreeBSD) iEYEARECAAYFAkzIRmoACgkQC3+MBN1Mb4hBfgCgj/TfCiP37gkAWduqkwKUcVUu znIAnAjXynCj/fREjD+isFv8JUSwMxko =dK3o -----END PGP SIGNATURE----- --37ZONDoqvyhZS3Cx--
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