Date: Tue, 5 Aug 2008 22:43:41 +0300 From: Kostik Belousov <kostikbel@gmail.com> To: Rick Macklem <rmacklem@uoguelph.ca> Cc: freebsd-fs@freebsd.org Subject: Re: doing vfs_hash_get when vnode locked Message-ID: <20080805194341.GI97161@deviant.kiev.zoral.com.ua> In-Reply-To: <Pine.GSO.4.63.0808051342290.2620@muncher.cs.uoguelph.ca> References: <Pine.GSO.4.63.0808041657200.3482@muncher.cs.uoguelph.ca> <20080805083229.GB97161@deviant.kiev.zoral.com.ua> <Pine.GSO.4.63.0808051052350.27663@muncher.cs.uoguelph.ca> <20080805153221.GG97161@deviant.kiev.zoral.com.ua> <Pine.GSO.4.63.0808051242110.23305@muncher.cs.uoguelph.ca> <20080805165114.GH97161@deviant.kiev.zoral.com.ua> <Pine.GSO.4.63.0808051342290.2620@muncher.cs.uoguelph.ca>
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--1BXV+/FYeXhtv2WT Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Aug 05, 2008 at 01:51:40PM -0400, Rick Macklem wrote: >=20 >=20 > On Tue, 5 Aug 2008, Kostik Belousov wrote: >=20 > [stuff snipped] > >>What does v_usecount mean then, if it doesn't say "I have it in use, so > >>you can't recycle it until I vrele() it"? > >It means that the vnode memory will not be freed until vrele(). > > > >But the VOP_RECLAIM may be called any time, and it requires exclusive lo= ck. > >After vnode is reclaimed, it is reassigned to the deadfs. In particular, > >VOP_RECLAIM implementation must clear v_data. > > > >For the reclaimed vnode you still hold a reference to, you can reliably > >obtain the vnode lock. > > > [stuff snipped] > >I do not know about these systems, esp. whether and how they implement > >a forced unmount. > > > Ok, I just spent a few minutes snooping around in vfs_subr.c and I think > I see the problem. vget() has called vholdl() and then=20 > v_upgrade_usecount(), which has incremented the usecount and taken the > vnode off the free list. This appears to prevent vgonel() from being > called on it for most cases, but there is still the case in vflush() > where the FORCECLOSE flag is set. Yes, exactly. >=20 > But, it seems that it is my nfs_unmount() that calls this, so I can just > delay the FORCECLOSE for this weird case. >=20 > In fact, it looks like vgonel() would call VOP_CLOSE() because v_usecount > is still non-zero (active) and that would block during the recovery in my > code, anyhow. But, what guarantees that the vnode would not be reclaimed before/under your vref() it ? For instance, what if the vnode is locked due to reclaim being in progress ? --1BXV+/FYeXhtv2WT Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (FreeBSD) iEUEARECAAYFAkiYrW0ACgkQC3+MBN1Mb4i3gwCXdpeU8gvT1AGkKgT2Eh2lGHcQ gACg7k7+/cXzve/72UvEDJlIfCy1ENs= =asfp -----END PGP SIGNATURE----- --1BXV+/FYeXhtv2WT--
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