Date: Sat, 17 May 2014 22:22:29 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Benjamin Kaduk <bjk@freebsd.org> Cc: freebsd-fs@freebsd.org Subject: Re: Add an assert that v_holdcnt >= v_usecount? Message-ID: <20140517192229.GA74331@kib.kiev.ua> In-Reply-To: <alpine.GSO.1.10.1405171326480.25244@multics.mit.edu> References: <alpine.GSO.1.10.1405171326480.25244@multics.mit.edu>
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On Sat, May 17, 2014 at 01:48:11PM -0400, Benjamin Kaduk wrote:
> jhb was helping me debug a crashy openafs build in one of my VMs, and the=
=20
> symptoms seemed to indicate that a vnode had been partially destroyed=20
> before vgone() was called from vflush(), as if some buggy filesystem code=
=20
> had called vdrop() instead of vrele() or something similar. In a quick=
=20
> check, it didn't look like we had any assertions that would catch such=20
> bugs, so I tried adding something like this:
>=20
> Index: sys/kern/vfs_subr.c
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> --- sys/kern/vfs_subr.c (revision 266330)
> +++ sys/kern/vfs_subr.c (working copy)
> @@ -2343,6 +2343,8 @@
> if (vp->v_holdcnt <=3D 0)
> panic("vdrop: holdcnt %d", vp->v_holdcnt);
> vp->v_holdcnt--;
> + VNASSERT(vp->v_holdcnt >=3D vp->v_usecount, vp,
> + ("hold count less than use count"));
> if (vp->v_holdcnt > 0) {
> VI_UNLOCK(vp);
> return;
>=20
> Does that seem like something that would be generally useful?
This is reasonable.
As a note, I never seen such corruption of the otherwise valid vnode state=
=20
ever. There were a lot of leaks, but never mismatched vget/vdrop.
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