doing vfs_hash_get when vnode locked

[Kostik Belousov]

On Tue, Aug 05, 2008 at 04:58:30PM -0400, Rick Macklem wrote:
> 
> 
> On Tue, 5 Aug 2008, Kostik Belousov wrote:
> 
> [stuff snipped]
> >>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
> >>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.
> >
> [more stuff snipped]
> >
> >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 ?
> >
> So long as I never do a vflush() with FORCECLOSE, I can't see anywhere 
> that will vgonel() it once I have gotten it via vget(). (v_usecount
> incremented and not on the vnode freelist)
> 
> The way I just coded it is:
> - the function that does the vfs_hash_get() without LK_EXCLUSIVE just
>   fails if MNTK_UNMOUNTF is set.
> - my nfs_close just returns when MNTK_UNMOUNTF is set.
> - my nfs_unmount() doesn't set FORCECLOSE on the vflush() but instead
>   sleeps and retries a bunch of times if vflush() fails for MNT_FORCE.
> - my nfs_unmount() and other code (mostly based on the vanilla FreeBSD
>   client makes requests all fail with EINTR when MNTK_UNMOUNTF is set).
You still has the race where the MNTK_UNMOUNTF is set after you check
returned false, isn't it ?

BTW, is your fs marked as mpsafe ?
> 
> I think this should work for a forced unmount, since once requests all
> fail and the recovery also fails, I think vflush() will work without
> the FORCECLOSE flag.
> 
> As far as I can see, since I'm not vflush()'ng with FORCECLOSE, then
> nothing will vgonel() the vnode until it has been vrele()'d. (If there
> is a case other than vflush() with FORCECLOSE that will vgone() it when
> it is not on the freelist and has a v_usecount > 0, then I'll need to
> handle that as well, but I can't see one.)

Yes, ATM it should be safe, since only vflush() does reclamation for the
vnodes with usecount > 0. On the other hand, I believe our VFS never
makes a guarantee that this is the only location of the call.
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