From owner-freebsd-hackers  Tue Sep  4  6:58: 8 2001
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From: Maxim Sobolev <sobomax@FreeBSD.ORG>
Message-Id: <200109041343.f84DhO097646@vega.vega.com>
Subject: Re: Junior Kernel Hacker task: improve vnode->v_tag
To: phk@critter.freebsd.dk (Poul-Henning Kamp)
Date: Tue, 4 Sep 2001 16:42:03 +0300 (EEST)
Cc: brent@rcfile.org (Brent Verner), current@FreeBSD.ORG,
	hackers@FreeBSD.ORG
In-Reply-To: <12839.999606420@critter> from "Poul-Henning Kamp" at Sep 04, 2001 02:27:00 PM
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> 
> In message <20010904081337.A8968@rcfile.org>, Brent Verner writes:
> >On 04 Sep 2001 at 10:36 (+0200), Poul-Henning Kamp wrote:
> >| 
> >| Assignment:
> >| 
> >| The v_tag element in struct vnode is a debugging aid, but unfortunately
> >| it is implemented in a way which means that adding a filesystem means
> >| modifying the definition in <sys/vnode.h>.
> >| 
> >| Convert the v_tag to an "const char *" and have the filesystems put
> >| their name in there instead.
> >| 
> >| The v_tag has been abused a few places, easily recognizable by the fact
> >| that the kernel should never inspect the value of v_tag.
> >| These places should be easily changeable to use the new representation.
> >| Please mark them with a big fat "/*XXX: ABUSE OF v_tag */" comment.
> >
> >#include <newbie_kernel_hacker_warning.h>
> >
> >I've done a /cursory/ look over how this v_tag is used.  I'm not sure
> >this is a simple/clean as you propose, since this is used in the 
> >IS_LOCKING_VFS macro, as well as in union_subr.c...
> 
> Well, that is just too bad, because IS_LOCKING_VFS is wrong then.
> 
> The places which inspect v_tag will have to be changed to use
> strcmp() then...

I think that we can add a new vnode flag, say VCANLOCK, so that each
particular VFS can set it if it supports locking, which should allow
to remove pre-defined VFS list from the IS_LOCKING_VFS macro. I can
produce a patch if it sounds reasonably.

-Maxim


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