[PATCH] Fix 'implicit declaration' warning and update vgone(9)

[Benjamin Kaduk]

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 at chaotika.org> wrote:
>>> The vgonel function isnt declarated in any header, the vgonel prototype
>>> in vgone(9) isnt correct - found by Ben Kaduk <kaduk at 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