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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[-- Attachment #1 --] On Wed, Oct 27, 2010 at 10:59:56AM -0400, Benjamin Kaduk wrote: > On Wed, 27 Oct 2010, John Baldwin wrote: > > >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) > >>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 the > >reporter is getting a warning since there is a static prototype for > >vgonel() > >in vfs_subr.c. > > It's for a third-party kernel module, for the OpenAFS filesystem (which > has been unloved for FreeBSD since the 4.X days). The AFS code is > currently using unlocked accesses to v_usecount (which, unsurprisingly, > led to a race condition that caused an invariant check to fail), and I was > going through and adding the interlock around them. At the place that I > suspect to be the main cause of this race [1], the usecount was checked to > be nonpositive along with a couple other conditions, and a little later > vgone() was called. Holding the interlock across both of these calls (and > therefore using vgonel()) seems to have closed the race condition I was > seeing. (Other checks of v_usecount were changed to grab the interlock, > but drop it before doing anything else.) > However, looking at the commit message for vfs_subr.c:1.631, I guess this > is not the locking strategy I'm supposed to be using? > > I saw a warning of implicit declaration when compiling the kernel module, > but the kernel linker was happy to load it. I forget whether it matters > that vgonel is only declared static at its declaration but not its > definition. > > -Ben Kaduk > > > [1] The old (racy) function is osi_TryEvictVCache, here: > http://git.openafs.org/?p=openafs.git;a=blob;f=src/afs/FBSD/osi_vcache.c;h=c2060c74f0155a610d2ea94f3c7f508e8ca4373a;hb=HEAD 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 refcounter. See the vfs_subr.c:vlrureclaim() for the correct dance with vholdl()/vn_lock() sequence that does the revocation I mentioned in the race-free way. [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (FreeBSD) iEYEARECAAYFAkzIRmoACgkQC3+MBN1Mb4hBfgCgj/TfCiP37gkAWduqkwKUcVUu znIAnAjXynCj/fREjD+isFv8JUSwMxko =dK3o -----END PGP SIGNATURE-----
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